Deep down dolphin

There was certainly plenty to see in the ocean; rock structures, reefs, shipwrecks, and a plethora of other things. It wasn't just divers from the land that would explore these, as even the natives enjoyed doing so. One in particular, a male dolphin, was feeling especially adventurous today, diving deeper and deeper, far beyond the point where the surface wasn't visible. He was almost 600 feet down and still going strong despite the growing water pressure, keeping his mouth and blowhole tightly shut, not having lost even a single bubble from either. It was getting quite dark at this depth, as he bordered on the bathypelagic zone, but that wasn't an issue for him; with some internal clicks, the world almost seemed to light up around him, his echolocation allowing him to "see" despite the lack of light.
Another couple hundred feet, and the water pressure was close to being too much for the dolphin. If he went another 100 feet or so down, it would start to hurt, but thankfully he was coming upon something below, marking that he was almost at the bottom: A shipwreck! He was lucky he'd gone straight down, while he was starting to feel the need to breathe he still had plenty of time before he'd really need to.
It didn't take long for him to find an entrance, the wreck having happened to land upright, and he floated in front of it for a moment, scanning the inside with sonar as everything beyond a few feet in was pitch black. The dolphin wasn't exactly an expert in determining the structural integrity of anything, but from what he could tell, it was reasonably intact. Without any more delay, he cautiously swam inside.
Now entirely unable to see, the dolphin relied on his sonar to explore, and the image from it seemed a bit clearer as he focused on it. He was in a fairly open room, with several doorways to choose from, only one of which had an open door. Thankfully, all the doors had handles instead of knobs, and it only took a few tries to get a good grip on them with his flippers, pushing open the door with his head.
This room had a toppled desk, writing implements and well-rotted papers strewn about. The papers likely had things written on them, but sonar wouldn't show them, and it wasn't like the dolphin could read anyway. With no real reason to stick around, the dolphin went back to the bigger room, getting through the opposite door with some effort.
There were a bunch of beds in this one, as well as a trunk of some sort, which he swam up to, putting his head against it and letting out some clicks to scan what was inside. It was hard to make anything out, but there didn't seem to be anything of interest. Once again, the dolphin returned to the main room.
The last doorway led to a hall, and it wasn't long before he found that it was blocked by broken wood and other such clutter. Not entirely, though; there was a small hole that seemed big enough for him to fit through, but not by much. The dolphin hesitated, somewhat worried he might end up getting stuck, and needing a breath as well, his chest starting to heave... But, too adventurous for his own good, he swam forward regardless to attempt to squeeze through.
With his flippers pressed against himself to get just a little more room, the dolphin tried to swim through the hole, the sides of it rubbing against his body... And eventually stopping him. His eyes went wide and he pumped his tail harder and harder, fear quickly building. Thankfully, he wasn't stuck for long, and shot out the opposite side of the hole, whining as the edges scraped him. Some bits of clutter were taken with him, and there was a brief loud creaking before the ceiling behind him collapsed, fully blocking his way out!
There was no possible way the dolphin could get back the way he came, now, and he'd run out of breath long before he'd be able to clear a path. All he could do now was continue deeper into the wreck and hope he'd find another way out. There was another door next to him, and while this one took more effort than the others, it was opened before long.
Inside, there were several piles of small circular objects, as well as a few differently-shaped ones of different density. He didn't know it, still unable to visually see anything due to the darkness, but he'd found a bunch of treasure! Of course, it wasn't as if he could pay his way out of the ship, and he had no concept of currency anyway, so he simply moved on.
The dolphin's growing fear was using up his dwindling oxygen more quickly, and his lungs were just beginning to burn. It was rare that he held his breath this long, and it seemed he'd have to hang on quite a while longer. Another door was found and pushed open, this one leading to a kitchen.
Various pot and pans lay on the floor, but oddly, one in particular was upside-down on the ceiling. After approaching it and trying his sonar again, he noticed most of the inside was a lower density than the water... It held air! But, no matter how the dolphin positioned himself, he couldn't get his blowhole or even his muzzle far enough in to get at the air. He whined, briefly curling inward as tiny bubbles began to trickle from his blowhole, its flap of skin starting to quiver. Something needed to be done, and soon!
Once again, the increasingly-breathless dolphin darted from the room, soon finding another and barging in. A bathroom, this time, with an overturned bathtub that had one side propped up on a cabinet, enough room beneath that he could fit in. The tub also had a few bubbles' worth of air in it, and that gave him an idea. Carefully, he swam under it, and after confirming he could get his blowhole to the air, started to exhale. A stream of bubbles was released from his blowhole, soon turning the tiny amount of air into a decent air pocket that he could breathe inside! He took shallow, careful breaths, constantly worrying about accidentally inhaling too deeply and sucking down water, but not needing to hold his breath for the moment was helpful.
After some time of rebreathing his stale air, the dolphin sucked in as much of it as he could safely get, and swam out from under the tub. While he now wasn't leaking any bubbles, he knew it would start again soon, and he needed to be ready. A quick scan of the room found him the tub's plug, and while it wouldn't be pleasant, he could some use of it. He scooped it up with a flipper after a couple tries, turning it upside-down, then went upside-down himself, lowering down and positioning himself so his blowhole was directly above the plug. Then, tightly shutting his eyes, the dolphin lowered further and carefully opened his blowhole as the plug slipped inside! It felt awful, the rubber sliding against the sensitive flesh in his airway, but with an experimental attempted inhale, it was clear that the plug was fulfilling its strange new purpose.
Now satisfied that he wouldn't be losing any more air, he righted himself and left the room. Each heave of his chest now briefly pulled the plug tighter against his blowhole, and he shuddered in discomfort. Trying to ignore it, he focused on what was ahead of him, that being a stairway to a lower level. Down here seemed to be one big storage room, with all sorts of boxes and barrels around. The dolphin scanned the inside of one, finding it had many hard spheres, and that much of it, like the pot before, had a lower density! He pried it open, hearing a cloud of bubbles escape, and darted above it, turning upside-down to inhale the air.
Except, he didn't. The plug shifted in his blowhole, and all the bubbles simply slid off him and dispersed along the ceiling as he cringed and loudly whined. The dolphin didn't have time to look for more air; the inferno in his lungs combined was causing him to start shivering, and he could only assume he was starting to turn blue from airlessness. Just after, though, he noticed a very subtle light, the first he'd seen in almost 10 minutes, coming from behind some barrels. He shoved them aside, wasting even more of what little oxygen he had left, and found that part of the wall was heavily damaged, enough that he could see outside!
The dolphin swam against the wall in an attempt to break through, only managing to bend its boards, and not enough to slip through. He tried again with more force, but it didn't make any noticeable difference. Then, he swam to the opposite wall, faced that weak section, and charged toward it, lowering his head at the last moment.
Crunch! He broke through the wall, and was outside again at last! Without any time to waste, he shot for the distance surface with no hesitation. Just 20 seconds into his desperate ascent, the dolphin's lungs finally gave up, forcing him to gasp through his blowhole... Or at least attempt to do so. The plug continued to sit snugly in place, blocking off the water and keeping his exhausted lungs dry. Again and again, he tried to inhale, repeatedly sucking on the plug as he began to feel dizzy.
Slowly, the light around him grew, and eventually he could see the surface again! Exhausted as he was from swimming so quickly for so long, the dolphin forced himself to keep going. His lips trembled, and his mouth opened slightly, seawater pouring inside before he managed to drink it to keep it from reaching his lungs. Each gulp took more and more focus, and as he began to dread yet another swallow, he finally reached the surface!
With a loud gasp through his mouth, he curled inward and pushed through his blowhole, shooting the plug out quite a distance. Panting heavily, he slowly recovered, returning to his normal coloration as he had indeed gone a bit blue. That dive had to have been his longest yet, though he didn't plan on doing another like it, as his poor lungs had taken quite the abuse from holding onto a single breath for so long. Soon after catching his breath, the dolphin dove back below, deciding to take a break from further exploring for today.