The aquatic assignment

Bubbles the penguin reported in to the agency he worked for, ready for whatever the assignment he'd been called in for was. The formalities were quickly taken care of, as his boss got straight to the point. "Another spy of ours has discovered the location of an enemy base, deep in the Pacific Ocean. All that's known is that it seems to be staffed solely by water-breathing species. We need you to get down there, infiltrate the base, and learn its purpose."
"...As good as my lungs are, sir, it's not going to be easy to hold my breath long enough for that..."
"I'm well aware, so I've gotten you some basic scuba gear. That should at least give you another few hours, on top of your normal breath hold if it comes to that. Besides that, remember that this is going to be a stealth mission. We don't know what the enemy will do if you're caught, so assume the worst."
"Lucky I'm fairly small, then... Anything else important?"
"Not particularly. You'll be guided to the exact position above the base tomorrow. Unless you have questions of your own, you're dismissed."
"Nothing to ask, sir. Here's hoping it all goes well." Bubbles left the room, thinking about his upcoming mission and what he might end up finding...

The next day, Bubbles was out on a small boat about a mile from shore. This was the spot... Theoretically, anyone who saw the boat without him in it wouldn't think much of it, so he shouldn't have anything to worry about in that regard. He checked over his scuba gear one last time - everything seemed fine. While he wasn't about to admit it, he was nervous about the mission, moreso than usual. He could handle himself well underwater, of course, but not as well as someone that was truly aquatic...
Steeling himself, the penguin put on the scuba gear, as well as a belt that held a pouch with a waterproof pen and notebook, and looked down into the water. There wasn't much sense in just waiting up here. With a practiced dive, Bubbles jumped into the water, briefly surrounded by his namesake before his surroundings cleared. After breathing in through the regulator, he began swimming down toward the base deep below.
About 5 minutes passed uneventfully as he swam, the sunlight dimming and dimming... Then, he could finally see just a little bit of light coming from below. That'll be the base, Bubbles thought as he continued. As it gradually came into better view, it seemed fairly big... He had a lot of exploring to do. Before continuing down, he headed to one side for a while, planning on getting to the bottom and using the seafloor for cover.
Bubbles successfully reached the bottom, starting to sneak his way toward the base. He could see movement fairly nearby, and not just of random passing fish - he could tell those were guards. A handful seemed slightly bioluminescent, so avoiding those ones wouldn't be as hard, but the others... Well, good thing he was good at staying unseen. This base must be pretty important, with so many guards around, he had to see what it held.
He continued on, very carefully tracking the routes of the guards, thankful for the scuba gear he'd been given... In about 10 tense minutes, Bubbles had gotten himself pretty close to the base's entrance-
"Hold it!" Bubbles froze up, apparently having been seen... He slowly turned to see who it was. It was an anthro great white shark! They were unarmed, but he knew they hardly needed weapons with how sharp their teeth must be. "A spy, huh? Ugh, I wish I could just tear you apart here and now, but I need to actually bring you in..."
Bubbles looked over the shark, rapidly taking in their appearance and trying to think of a plan. Hitting their eyes likely wasn't going to happen, and just punching them was basically impossible underwater... But, looking at their gills gave him an idea. He put his hands up as if he were surrendering, and when the shark went to cuff him... He bolted forward and up, jamming a flipper into their gills!
"GHK!" The shark immediately slapped Bubbles' flipper away and clutched at his gills, sent into a coughing fit, while Bubbles swam away as fast as he could! He had to get into the base, he could find a hiding spot in there...
"Y-you little bastard, COME HERE!" The shark gave chase, rapidly catching up with the penguin. Just as Bubbles passed through a door to get inside the base, the shark gave a powerful bite, just barely missing him... But not his air tank. There was an explosion of bubbles, knocking the two in opposite directions as Bubbles instinctively gasped and held his breath. While the shark was disoriented, the penguin bolted out of view, darting through the base's halls until he felt confident he'd lost them.
Bubbles took off his scuba gear and looked it over... It had been completely ruined. He shuddered as he looked at the tank, nearly bitten clean in half. If that bite had been just a little closer... Putting that thought aside, he set down and hid the now-useless scuba gear. All he had now was his lungs... But he couldn't just leave without doing his investigation. With his breath holding record sitting at a good, if very uncomfortable to achieve, 30 minutes, he had plenty of time. After looking out from his hiding spot to make sure no one was around, he took off swimming through the halls. He'd been trained to have very good control of his breath, and it showed - there wasn't even a single bubble being left behind to mark his path as he swam, and he knew it'd be a while before that started to change.
As he swam, he soon came to a map on a wall, and he paused to look it over. There were 3 things marked on it that interested him: "Material storage", "Weapons research lab", and "Subject containment". Those subjects must be for testing the weapons on... Notably, there was no mention of air anywhere. Bubbles noted down the directions for the route he had planned: Materal storage, then the lab, then the containment. That would definitely give the sort of information his ageny was looking for... Hopefully he'd have the lungs for it. Just in case, he also noted down escape routes going from each of them.
Bubbles resumed swimming. There were very few cameras, so he didn't really have to worry in that regard, but there were certainly people around. Every now and then, he'd have to duck into a side room, or locker, or what-have-you, to wait for someone to pass. It was annoyingly cutting down on the time he had, but it was a necessary evil - remembering the subject containment room, he had an idea of what would come if he got captured.
While it wasn't far, Bubbles had been holding his breath for about 6 minutes when he finally got to the storage room. He carefully peeked inside... There was an anthro marlin in there, writing something at a desk, thankfully facing away. The penguin slowly moved into the room, hid to make absolutely sure the marlin wouldn't be seeing him if they looked his direction, and started to look around. It was certainly tough, but with the marlin clearly in no hurry to leave, this was Bubbles' only real option.
There were certainly quite a few things in the room: Spare parts, bars of metal... What caught his attention most were things that looked like cubic crystals. Bubbles quickly took some notes on them. Cubic crystals, unknown use. Box labelled as "Luxium". Bubbles put one of the crystal things in his pouch, before he heard movement! Peeking out just a tiny bit, the penguin saw that the marlin had gotten up and was looking around the room, hopefully doing inventory. Bubbles watched them, and as they kneeled down to inspect the contents of a crate, he bolted for the door, just barely managing to get out without being seen.
That was one down... He was getting nervous, feeling the urge to breathe by now but still not being done yet. Next would be the lab... Referencing the directions he'd noted down - and very thankful he'd done so - he took off swimming through the halls again. It didn't take quite as long this time, but even still, he'd been holding his breath for 13 minutes by the time he got there.
He peeked inside again, and a little smile formed on his beak as he didn't see anyone. After swimming in and shutting the door, Bubbles ignored a heave of his chest as he looked around, seeing some odd machines around and quickly noticing some plans laid out on a table. He inspected them... Sure enough, it was for a weapon of some kind! The notebook and pen came back out, and he sketched what the weapon looked like according to the plan, as well as some notes about what the function seemed to be. "U-ASMD": Fires lasers, and floating balls of energy that can be detonated with said lasers. He knew taking the plans would be an awful idea, so he put his notebook and pen away and went to move on.
But, as Bubbles approached the door, he could hear voices on the other side getting closer! He frantically looked around for a hiding spot, darting into a locker and quietly closing its door just as the room's door opened. An anglerfish and an eel, both anthro, swam into the room, in the middle of a discussion of how they might go about making the gun whose plans he'd seen a reality.
The penguin couldn't remember the last time he was in this bad a situation. As the two researchers tended to wander the room unpredictably in their discussion, trying to leave would be a very risky thing. His best option, as much as he hated to admit it to himself, was to wait and hope they'd leave in time. For now, he made the most of the time and listened in, ready to make notes of anything particularly interesting the researchers said.
Minutes ticked by, and the two just kept talking, occasionally checking notes they had. Bubbles had been holding his breath for 19 minutes, 20, 21... This was past his comfortable limit, and he was having trouble focusing for two reasons. His need to breathe was getting worse and worse, and bubbles were beginning to leave him. The other reason though, was actually kind of nice. Part of the reason Bubbles had so much practice holding his breath a long time was because he liked the exact feeling he had right now, and he felt himself starting to get erect.
As much as he didn't want to masturbate - he had to take this seriously! - over time, Bubbles was gradually getting more aroused from his growing breathlessness. The researchers weren't leaving, so he couldn't go anywhere, and he started to grunt and whine at the 23 minute mark. He kept quiet as best as he could, covering his beak and hoping the locker would muffle him the rest of the way. The very beginning of a tingly burning began to form in his chest, and he looked up at the bubbles that had pooled at the top of the locker. C-couldn't hurt... He swam up and carefully pulled the air back in, and while it didn't improve anything, he felt a little better... And it further added to his arousal.
Another minute, and Bubbles just couldn't resist any more. Taking one flipper off his beak, he started to masturbate, letting out a muffled moan before he stopped himself. That sound had apparently been heard by the researchers... But they didn't seem to think much of it, chalking it up to some random metallic groaning. More and more time passed, the penguin's face starting to gain a purple blush as he began to consider just leaving his hiding spot and hoping for the best.
One of the researchers started up a machine of some kind, its whirring filling the room with noise that would help cover the penguin's struggling. He didn't hold himself back as much, somewhat relieved to let his breathless groans out without needing to really worry. Bubbles even released some air to relax his lungs, being very careful to not let any leak out of the locker he was still stuck in.
Over time, Bubbles was getting closer to an orgasm... As odd as it felt to be doing this during a mission, it was helping him stay calm as he got to 27 minutes. He knew at this point he likely wouldn't be surfacing with dry lungs, even though he was now planning on skipping the subject containment... But he could handle some water breathing. Wouldn't be his first time.
He let out another light moan as his arousal surged... And Bubbles came into the water, balancing out his strain with pleasure. While that was still ongoing though, the machine covering his noises stopped, and the penguin had to quickly cover his beak to hold back another moan, his orgasm continuing on hands-free for a few seconds. As he came down from it, he began to shiver from his need to breathe... But finally, mercifully, the researchers left the room, giving him a chance to get out!
Quickly leaving the locker, he swam to the room's door to make sure the two fish were leaving, which they were. Keeping a flipper on his beak to not let out any bubbles, the penguin checked his notebook for the escape route he'd written down, and began to follow it. He tried his best to keep the grunting and groaning to a minimum as he rebreathed in his puffed cheeks, dreading the inevitable breath he was going to be taking so far underwater...
28 minutes, 29... 30. Bubbles' body was shaking all over, his blush now a deep blue. His cock was throbbing in arousal again, even though he wouldn't be cumming again so soon. While he tried to keep moving, he froze up as the inferno in his lungs became too much to handle. After a moment, he thrashed, starting to exhale clouds of bubbles despite his best efforts... Before water surged down his throat.
Despite himself, Bubbles let out a few loud, pained gurgles as he began to choke, coughing up his useless air and gasping down water over and over. Not too long into that, he could hear something... Someone was coming! He darted into a nearby closet, as much as he wanted to frantically swim for the exit, and watched through it as he tried his best to keep quiet. Grabbing his neck, he still let out pained whines and little slurps despite his best efforts. D-damn it, SHUT UP, they're gonna hear me! Just before the person out in the hall, a thresher shark, came close enough to hear, Bubbles took in a deep watery breath and held it!
The penguin's blue cheeks puffed up with water as he fought with everything he had to keep quiet. Outside, the shark looked up at the exhaled air that had pooled up on the ceiling, confused. "...We don't hire air-breathers... Damn it, INTRUDER!" The shark bolted away just before Bubbles' will broke and he hacked up the water he'd been holding. After checking to make sure they'd gone, he left the closet and continued his escape.
He was getting increasingly lightheaded as he frantically swam, still coughing and gasping as he went into the second minute of his drowning. As the oxygen in his blood was used up, the pain of drowning was fading, and he eventually managed to just heavily pant instead of choking. Parts of Bubbles were tingling, losing sensation as he weakened over time... But, worst of all, as he turned another corner, he saw the exit he'd been planning on taking was under maintenance! There were too many people around it, he had to go somewhere else!
As Bubbles searched for another way out, the second minute passed into a third. His swimming was getting sloppy now, darkness beginning to move in from the edges of his vision... A hiccup helped him keep himself aware as he continued on. At this point, his dulled mind considered just letting himself be caught. Maybe the people here had air for their subjects?
But, for better or worse, no one came, and a fourth minute began. The waterlogged penguin was in a sorry state now, lacking the coordination to swim any more. He sank to the floor and stumbled along, leaning against a wall to keep himself propped up. There wasn't a whole lot he could feel any more, mainly just his erection and his slow, weak heartbeat... Before he tripped, falling to all fours. Bubbles breathed heavily for what it was worth, his vision dimming and sensations fading... Then it occurred to him he couldn't feel his heartbeat any more. He had so little oxygen, so little energy... His heart had stopped from how drowned he was.
Bubbles wasn't going to let this be his end. He focused what little he could, kneeling on the floor, and beat his flippers against his chest with what strength he had left in an attempt to restart his heart. His energy was fading even faster than before, and each hit made him hiccup... On the fourth hit, he gasped and coughed a few times as his heart jolted back into motion, adrenaline rushing through him and letting him swim again, which he immediately took advantage of.
The adrenaline rush let him get into a fifth minute of having flooded lungs, and he was closing in on a safe exit at last! But, his energy was rapidly draining again, and the thoroughly-drowned penguin soon collapsed onto the floor again. Like before, his heart stuttered to a stop, giving out from the extreme lack of oxygen, and he began to beat on his chest again. He could feel his flippers getting heavier with each hit, taking more and more effort each time... And as much as he kept trying, he was soon too weak for another hit. He did everything he could to force himself to gather his strength, but he just couldn't any more. This was it...
Bubbles blankly stared down at himself, his cock still fully erect... He managed to just barely make himself twitch, trying to reach to masturbate again, but it wasn't happening. His vision faded more and more... Then, it went to nothing. The penguin was fully drowned, limp and unconscious, body still inside and out.
About 20 minutes passed with him kneeling there, far beyond the point of saving, before he was finally found. None other than the great white shark that'd spotted him earlier swam up to his body. "Heh, how the mighty have fallen..." The shark lifted him up by a flipper, his eyes still open and staring at nothing. "If only you hadn't been reported... Never tried penguin before. Ah well, can't really be helped." The shark draped Bubbles' body over one shoulder and began to swim away, wondering what sort of reward he'd get for finding them.