Shifting below the waves

Out on the ocean one day, a quite out of place raven flew just over the surface, looking down in the water as he most likely searched for a meal of some kind. Eventually, he drew a few deep breaths and got some height, before holding his breath and diving toward the water. This strange raven quickly turned out to be even more strange as he started changing; his feathers and talons withdrew into his body, his beak softening and skin turning grey as his body grew... In seconds, he'd become a dolphin, revealing himself to be a shapeshifter!
With a splash, he dove underwater, and he smiled and swam deeper. He didn't have much air in his lungs, having begun holding his breath when they were a lot smaller, but this dive was more to see what was around. Soon, he'd gone 150 feet down, the surface only barely visible. He'd hit the jackpot of aquatic playgrounds; there was a reef on the seabed, with all sorts of aquatic life around for him to become and hang out with, and there was even a small shipwreck nearby!
"Mmg..." The shapeshifter's smile faltered a little as his chest heaved, bubbles spurting from his blowhole. Having so little air wouldn't do, so he started swimming back up. With this aquatic form, it didn't take long to reach the surface again, and he gasped through his blowhole as he thought about what to do first. He decided he'd mingle with the life down there for a while, then come up for a breath before exploring the wreck.
Remaining as a dolphin for now, he gasped again and swam back down to the reef. Now, what group to join first... Well, there was a small school of bluefin tuna about, so with a couple seconds of work, the shapeshifter turned himself into one. At this point, a limitation of his abilities was clear; try as he might, he couldn't figure out how to make working gills. So, he continued holding his breath, and as his lungs shrank to fit in his new body, his cheeks bulged out with the air that they couldn't hold any more. It looked rather silly, a fish holding their breath, but the shifter didn't mind.
When he approached the school of other tuna, none seemed to care about his cheeks, letting him join them. It was always a joy to swim in a school of fish, all the glimmering scales around him... Even being the only one that couldn't breathe, it was fun to be in a group like this.
He happily swam with them for a couple minutes, before they started to leave the reef. Not wanting to do the same, he waved a fin to say goodbye - fully knowing they wouldn't understand - and left the group, swimming back to the reef. With his strange fish lungs still securely holding in his air, the shifter looked around for another group to join for a while. He had a bit of a hard time deciding this time, but after seeing some crabs on the seabed, it occurred to him that he'd never made himself into a crab before.
Swimming down to the seabed himself, his scales hardened into a shell, several legs extended out beneath him, and his fins moved forward as they became pincers. The process shrank him a little, and his cheeks inflated bigger, but he still kept his air in. Now fully a crab for the first time, the shifter experimentally clicked his pincers, getting a feel about how to use them, and stumbled a little on his collection of legs, not used to having so many. Of course, being a natural shapeshifter, he had a knack for figuring out how to move in different bodies, and within a minute he was scuttling around on the seabed as if he'd always been a crab.
After climbing around in the reef for a couple minutes, he decided to just relax for a while, settling onto a large rock. He needed air, but not enough that he cared yet, and for now he enjoyed that feeling. There was the occasional plup of a bubble, or an "Mm" or "Mrk" of strain from the big-cheeked crab as he waited, his need to breathe gradually getting worse. Eventually, as the bubbling slowly increased, he held his pincers to his mouth to try to cover it, his cheeks continually puffing out a little more and sucking back in as he rebreathed.
With the shifter soon having been underwater for 10 minutes, he decided it was about time he got air. Knowing that swimming as a crab would be slow, if he even could, he transformed again; his shell softened into skin, and his limbs sank into his elongating, narrowing body as he turned into an eel, the first thing he'd thought of. What he hadn't thought of though, was how little room there was inside him for lungs.
His cheeks inflated even bigger, and bubbles spewed from his mouth as they reached their limit, quite a bit of air being wasted. Grumbling, then whining, the shifter began swimming for the surface. He still at least had a decent amount of air left, but his lungs were starting to burn, and eels aren't exactly known for their speed. Still, he stayed in this form, challenging himself. More bubbles started to dribble from his lips as his face turned a bit purple, and while he started getting aroused, his current body was genderless - as he always defaulted to - so nothing was visibly happening in that regard.
As the trickle of bubbles turned to a stream, the shifter surfaced with a gasp. As much as he enjoyed pushing himself, the first gasp after a long dive always felt great. He smiled as he caught his breath, and he looked down roughly where the shipwreck was, knowing he'd better switch to a form that can hold more air. His body began to widen and thicken again, scales appearing along it as a snout and 4 legs emerged.
Now an alligator, the shifter was more confident, and after some deep breathing he held his breath and started swimming to the shipwreck. This form didn't seem to quite do the best with the water pressure, but he got back to the wreck without issue, letting himself sink onto its deck to walk instead of swimming. He already knew there wouldn't be a whole lot to see, but it was interesting to look around something so old anyway.
After about a minute of strolling around on the deck, he found a way to get inside, which he took without hesitation. With all the furniture and other objects strewn about, it was a bit tough to just walk, so he started swimming instead, looking around for anything interesting. Along the way, the shifter got a little tired of his alligator form, so he changed again, his body becoming smaller and less threatening-looking as he became a marine iguana. The loss of size made his cheeks bulge quite large again, but he managed to keep in his air.
Another minute or so, and his progress through the ship was blocked by debris. Through a small opening, he could see another room, but he'd have to get pretty small to fit. That would mean losing quite a lot of air, too... If he was unprepared, at least. Close by was what had likely been a dining room, and he swam in to see if he could find something that would help him. It didn't take long; he found a nearly upside-down bowl with some air caught in it. That would be perfect!
The shifter picked up the bowl with his mouth and swam it over to the blockade, doing his best to prop it up. Then, after sizing up the hole again, he moved his head under the bowl as he changed again. He became much smaller this time, his body lengthening again, and air quickly started bubbling away to be caught in the bowl as he became a tiny snake. Once he'd finished transforming, he realized his huge cheeks were going to keep him from fitting, so he bubbled out some more air into the bowl, nearly filling it entirely, before swimming for the hole in the debris.
Fitting through was easy at this size, and it didn't take long for the shapeshifter to reach the other side, finding himself in what had likely been the captain's room. It was just as trashed as the rest of the ship, and while there was likely treasure to be had, he'd never really cared about that. What actually caught his attention was a cracked mirror, still quite usable because of his current small size. Both his snakey self and his reflection smiled at each other, and the shifter decided he could take a break for a little while to enjoy himself.
His body shortened and thickened, limbs returning with webbed hands and feet as he took his favorite form; a small anthro red-eyed tree frog. He finally gave himself a gender as well, a cock and pair of balls forming between his legs. He wasn't fond of most anthro forms, but this was one of the few exceptions. With a happy ribbit, he stared at his reflection as he started to jack off, finding this form as hot as ever. His small size meant he wasn't using much oxygen, but without much air in him, he was already feeling a need to breathe.
Just a minute in, he was struggling, little bubbles leaving his nostrils more and more frequently. The shifter rebreathed in his cheeks, even more elastic in this form than they were with others. The lack of oxygen was amplifying his pleasure, so even as he started getting a slight purple blush, he stayed where he was, though he pressed a webbed hand over his mouth to keep his air in.
Another 30 seconds, and the shifter was starting to shiver, his face beginning to turn blue as his lungs burn. But, at the same time, his arousal started to surge... And soon, he came, starting to shoot cum into the water. With a mix of a moan and a whine, he pressed his other hand to his mouth as well, kicking in an intense struggle to keep his breath in as his cock throbbed and spurted. Despite his small size, it was quite a large load, courtesy of being a shapeshifter; he always had more cum to shoot.
Too soon, the shifter couldn't keep holding their breath any more. As he orgasm slowed to a stop, bubbles erupted from his mouth as he instinctively tried to swim up... Then, he was stunned as water surged down his throat, into his lungs. He floated there with wide eyes and a blank expression for almost 10 seconds, holding in this aquatic breath and starting to sink, before his mind caught up with what was happening. Immediately going into a coughing fit, the shifter gurgled and choked, starting to swim to the debris he'd entered the room through.
Without thinking, the shifter tried to just force himself through the hole, though his frog body was too big to fit, bumping against the edges of it. Trying to focus past the searing pain in his throat and chest, he turned himself into a tiny snake once more, shuddering before beginning to slide through the hole again. The blue of his blush begins to slowly spread to the rest of his face as he hacks out the last of his bubbles.
After getting through, he frantically swims to the bowl of air he'd set up, hoarsely gasping before starting to cough up the water in him. He returned to his marine iguana form during this, his greater size allowing him to more effectively get the water out of his lungs. Once they were finally dry, he focused on remaking his strained, sore lungs and throat... And in seconds, the pain was almost entirely gone, the damage the water had done now reversed, the shifter very much glad of being able to rapidly heal by turning into a form he's already in.
Satisfied with both his exploration and playing in the reef, he decided it was about time he headed for the surface. Taking in as much air as his small reptile lungs could hold, the shifter started swimming back out the way he came. He got back outside without incident, and decided he may as well have a last bit of fun challenging himself on the way up. Feathers sprouted from his body, his muzzle hardening into a beak and his front legs moving as they became wings... Then, he was a raven again just like earlier, standing on the deck of the shipwreck with puffed cheeks.
He'd never swam as a bird before, and figured this would be a good time to try it out. With a jump, he instinctively tried to fly, but that certainly wouldn't be working here. Instead, he swam much like a typical biped, stroking with his wings and kicking his talons. That worked quite a bit better, despite ravens not being particularly good at swimming.
Still, one thing he couldn't help with was this body's breath holding ability. About 50 feet up, he was feeling the need to breathe again, but he was confident he'd be able to surface both with dry lungs and without changing forms. Another 50 feet, and he started to whine a little, blushing as bubbles start trickling from his beak. Eventually, the shifter's blush started turning a bit purple again, his beak starting to tremble...
And, just as a hint of blue formed on his face, he surfaced, gasping heavily and panting with as much of a smile as a bird can give. Forcing himself to hold his breath close to his limit was fun, but he'd had enough for now, wanting to head back to shore. Since he hadn't figured out how to fly from the water's surface yet, he changed forms one more time, deciding to push himself in a different way. His feathers, beak, and talons sunk into him as his body became much bigger, wings turning to flippers and a long, powerful tail extending behind him. With some effort, the shifter fully turned himself into a blue whale, gasping one more breath through his blowhole before diving under, slowly swimming toward the shore.