Trespassing in the depths

Several miles from any land, a lone boat steadily progressed farther and farther out into the ocean. Its pilot and only occupant was a riolu, a small bipedal Pokemon reminiscent of some sort of blue canine. Looking rather excited, he turned off the boat's engine and picked up his goggles, carefully putting them over his eyes. He'd been looking for this place for some time; if he had his position right, there were ruins deep below on the seabed, just waiting for an adventurous diver like him to explore them! With a deep breath, and some extra air stuffed into his cheeks, the riolu lept from his boat and splashed into the water.
For a moment, he was surrounded by bubbles, obscuring his vision, but it quickly passed. Wasting no time, he adjusted himself to look down, and indeed, he could see the ruins! They were at quite the depth, maybe about 150 feet, but the riolu was undaunted by the sight, and began swimming down to reach them. The water pressure slowly became more noticeable, though it was far from being an issue, even as he went below the tallest of the ruin's structures. After about a minute of swimming, with just a few feet left to go, he flipped himself rightside-up and allowed himself to sink the rest of the way, his paws settling down onto the bottom. He'd finally arrived! The thought that his bubbles were likely the only air this place had seen in potentially centuries made him smile and wag his tail, before he kicked off the bottom again to resume swimming and begin his exploration.
This place seemed to have been a large temple of some sort, though the riolu was very much underequipped to find out much more than that. Even what writing he could find, if that was even what it was, was essentially meaningless to him. Still, he enjoyed himself, and took in the sights for as long as his lungs were comfortable with, which was about another 4 minutes. With the urge to breathe reaching the point that it was distracting, the riolu started to head back up, the distance feeling quite a bit longer this time. He had no issues in surfacing, and gasped to refresh himself. Of course, being far from being satisfied with what little he'd seen of the ruins, he didn't remain at the surface for long, soon diving back down to continue exploring.
Feeling like pushing himself a bit this time, the riolu stayed underwater longer than before. Over the minutes, he bubbled increasingly often, his cheeks slowly expanding. 8 minutes in, with his cheeks about half the size of his head, he decided that was enough time, and started back to the surface. He hadn't really factored in how long it would take to get there, his chest beginning to heave and the bubbles that leaked from his nostrils starting to be in small clusters. Still, it would be quite a while before he'd reach his limit, and he didn't have much trouble resurfacing again, panting heavily afterward.
The riolu certainly planned on making another dive, but before he could, there was a voice behind him. "So." Startled, he quickly turned to see who it was. They were mostly submerged, with only their large silver-colored head above the water, strange blue spikes around their eyes. It was Lugia!
"What's this about, then?", they continued, staring expectantly at the riolu and looking rather grumpy.
"U-um, I... W-was just e-exploring...", he stammered out, rather intimidated at Lugia's size. Even their head was bigger than his entire body!
"The temple below us, and the waters surrounding it, are under my protection. You are trespassing."
"S-sorry, I didn't know! I'll leave, I'll leave!" The riolu started to swim back to his boat not too far away, but was quickly grabbed by Lugia and held in place.
"It's too late for that. Now that you've come here, I can't allow you to live. Take a deep breath if you wish, it will only delay the inevitable."
The riolu quickly gasped the deepest breath he could, but didn't have time to fill his cheeks with extra air (though he puffed them anyway) before he was pulled underwater! Lugia swam down incredibly quickly, even while holding the riolu, and in seconds they were both at the bottom. The riolu was then brought up to Lugia's face, who spoke again, the bubbles pouring from their mouth as they did revealing they too were holding their breath. "Chblosebln tblo hblold yoblur brbleatblh? Fiblne, leblt's seble hblow loblng yoblu lablst." 
The riolu squirmed in their grip. "Leblt gblo oblf mble!", he bubbled, his cheeks somehow not deflating in the process. After a few seconds of consideration, Lugia complied. "Dobln't swblim ublp, oblr I'lbl grblab yoblu agblain", they said.
Being able to move hardly helped the riolu's nerves, and he continued begging. "Pbl-plbleasble, Ibl'm soblrry! Iblf Ibl'd knblown, Ibl wobluldbl've stblayebld awblay!"
Lugia shook their head. "Noblt mbly prbloblblem."
As time passed, the riolu tried a few more times to convince Lugia to let him leave, but nothing seemed to work. He actually started to get a bit mad, since there didn't seem to have been any possible way for him to know to stay away. Regardless, his oxygen was slowly depleting, and while he'd been distracted from that for some time, his need to breathe made itself known again in the 9th minute. The riolu whined as his cheeks expanded like before, and he plugged his nose to help keep his air in.
"...Yoblu knblow whblat?", Lugia blubbed. "Ibl feblel liblke giblvinblg yoblu abl chblancble. I'lbl leblt yoblu leblave, iblf..." They poked at the riolu's cheeks, causing some bubbles to puff from his lips. "...Yoblu mablke thblose liblttlble chbleekbls oblf yoblurs hblalf abls wiblde abls Ibl ablm tabll."
The riolu looked at them in confusion. That didn't seem any better, Lugia was huge, about 17 feet tall! He at least gave it a shot, puffing his cheeks much bigger, about the size of his head. Lugia shook their head. "Unbless yoblu wablnt tblo brbleatblhe wablter, Ibl sublggeblst trblyinblg hblardbler." He put more effort into it, getting his cheeks even bigger... Eventually, with them growing to be 2 feet wide, any further air the riolu tried to force into his cheeks just spurted out his mouth in a cloud of bubbles, nothing more being able to fit into them. "Thbl-thblat's...Abl-abls biblg as... Thbl-thbley geblt...", he managed to say, straining to keep his cheeks at that size.
"Webll, thblat's unblforbltunblate. Yoblu mably abls webll jublst brbleatblhe, thblen."
The riolu sucked some of the air in his cheeks back into his lungs. "Abls iblf yoblu cobluld dblo anbly bebltteblr!", he yelled.
Soon, the 11th minute came, and with his cheeks back at the size of his head, the riolu started to blush from both the strain of holding his breath so long, and something else. He had a fart building up, demanding to be released, but he held it back, not wanting to embarrass himself. Although, he was likely done for, so what did it matter? Changing his mind, the riolu allowed himself to fart.
Prblrblrt! Bubbles streamed from his butt for almost a full second, eliciting a bubbly chuckle from Lugia. "Mablybe yoblu oublght tblo brbleatblhe thblose ibln? Siblnce yoblu're sblo inbltenblt obln lablstiblng sblo loblng." The riolu glared at them but didn't say anything, more concerned with keeping his air in for the moment.
The time was really starting to drag, now. The riolu was visibly struggling, emitting grunts and whines as he couldn't help but bubble out his stale air, punctuated by an occasional stress-fueled fart. 13 minutes in, he instinctively tried to swim up as his blush turned purple. Despite his skill at swimming, he barely got anywhere before he'd been grabbed by Lugia again. "Tobld yoblu noblt tblo swblim ublp!" He looked at Lugia with fear in his eyes, wriggling in their grip. The way he'd been grabbed had pinned one of his arms, but also covered his rear, preventing him from farting, which was slowly becoming an issue as gas was continuing to build up regardless.
Yet another minute passed, and the riolu grew desperate, face turning blue and each of his cheeks becoming almost 3 feet wide. With burning lungs and a cramping belly, he gave one more shot at convincing Lugia to let him go. "Pbl-pbleasble...Ibl-Ibl'm goblnna brbleatblhe!"
"Anbld ablout tiblme, toblo. Gblo obln."
Once again, the riolu's pleas fell on deaf ears, and he covered his mouth with his free hand. Bubbles streamed past his fingers as his cheeks started to deflate, and he could feel his belly bloating from all the gas he needed to fart out. In one more last-ditch effort, he moved his hand away to start punching at Lugia's, though it clearly wasn't accomplishing anything.
"BLBLPH... LBL-LEBLT M-MBLE... HMGRK... MMGHHUUUAAAGHK!" After almost 15 minutes underwater, the riolu finally had no choice but to inhale, sending a flood of water surging down his throat. For the moment, he held it in and puffed his cheeks again, mentally stunned. Before long, his mind caught up with the fact that he was drowning, forcing him to cough up a cloud of bubbles and take another desperate gasp.
"Fiblnalbly. Toblok yoblu loblng enblougblh", Lugia bubbled, and released him.
BRLRBLRBLBLRT! A cloud of bubbles immediately erupted from the riolu's newly-unblocked butt in a huge fart that lasted several seconds, quickly deflating his belly back to its normal size. He almost didn't notice, too caught up in his attempted aquatic breaths to care about anything else, frantically kicking about with both hands on his neck. After another few seconds of choking, he made to swim up, and Lugia didn't stop him this time, slowly kicking to follow him.
The drowning riolu clawed at the water, too panicked to swim properly, and combined with the water in his lungs weighing him down, his progress was far slower than before. Even now, he'd occasionally fart, the bubbles from his ass now being the only ones leaving him. It took a little over a minute and a half, but he managed to get just a few feet away from the surface-
Before Lugia grabbed him again and stopped him! He flailed about as best as he could, his desperate begging to be released being made unintelligible by his constant gurgles and hiccups, to the point that Lugia couldn't even tell he was trying to say anything. "Sblo clblose, hbluh? Hblere, I'lbl leblt yoblu tobluch iblt." Lugia lifted the riolu up some, allowing him to get his arms mostly above the surface but with his head still below, while also raising their own head up to breathe. Being underwater for 16 minutes had made even them run a little low on breath! They also took a moment to laugh at the riolu's flailing arms, then held their breath again and went back under, swimming down and dragging the riolu with them.
Again, the two returned to the seabed, and the riolu was released. He was in a rough state; while he wasn't choking any more, he still panted the water while awkwardly paddling, hardly getting anywhere. Eventually, he held a breath of water and puffed his cheeks again, each one being about 3 feet wide and visibly growing. Strangely, his belly had grown as well, filling with more water each time he'd taken a breath.
After a while of watching him drown, Lugia grabbed the riolu again and held him sideways, checking the width of his cheeks against their body... Indeed, they came up to half their height. They hadn't really been expecting him to actually manage it, but he had.
The riolu was brought up to Lugia's face again. "Hbley, wablke ublp. Ibl'm leblttiblng yoblu gblo, evblen thblougblh Ibl dobln't wablnt tblo." There was no response, the drowned riolu simply continuing his ineffective swimming and resuming breathing water. Lugia let go of him, and he quickly sank to the seabed, weakly hopping up and down in an attempt to get to the distant surface. The fact that he was still moving after drowning for 3 minutes was impressive, but he clearly wouldn't be surfacing on his own. Lugia decided to wait a while before helping him, sitting down and watching his fading struggles.
He didn't have much fight left in him, having gone mostly numb and somewhat delirious, even his intermittent farting having finally stopped. As his energy dwindled and his belly filled further with the water he was slowly breathing, his tiny hops got less and less height until he didn't even leave the seabed at all. Still, he kept trying as if it was working, and eventually lost his balance and fell on his back. The riolu's belly was a couple feet wide at this point, and with the very last of his energy beginning to run out, his lidded eyes began to grow unfocused, breathing slowing even further...

The next thing the riolu knew, he was back on his boat, on his hands and knees, hacking up water in the worst coughing fit he'd ever had. He could barely manage a hoarse gasp between each cough, but over almost a minute, he did manage to recover, though he still breathed heavily.
"You done?"
He looked up to see Lugia staring at him, looking somewhat less grumpy than they did earlier but clearly still not much caring for him. "...Y-you saved me?"
"As much as I probably shouldn't have. But, told you I'd let you leave if you got your cheeks big enough, and you did. I always honor my word."
There was a moment of silence. "Um... Could you maybe tell me about those ruins down there?"
"Don't push it, you're already lucky to be leaving here alive. Now go away and don't come back; if there's a next time, you won't be waking up."
"S-sorry, figured it was worth a shot!" The riolu turned and started up the boat's engine again, beginning to pilot it back to the distant shore. Lugia watched him for several minutes to make sure he didn't try anything, before finally diving back underwater, satisfied that they'd successfully protected the ruins.