Quite enjoyable! The way the sounds became muted underwater and the darkness when you got to the depths helped create a really lonely, foreboding atmosphere. Even without the pressing need for air, I still felt uneasy exploring for secrets and the like.
I appreciated how your stuff would pop out of you when you died, letting you collect it again if you could get to it - and the plentiful resources near the start to make sure you're not left helpless - but I ran into some issues with my dropped loot's collision:
It's hard to make out in the dark, but I died in a narrow spot to a bunch of leeches/eels and had so much stuff that it lagged like crazy and pushed my body out to the left of the room's walls. And then respawning got me this:
Slightly beneath the island. I was able to starve to death and properly appear on the island, but the game was so bogged-down by my old inventory still stuck in that little cave that I couldn't even fight off the first kind of vicious fish without dying. After a few failed attempts to get back down there, I had to give up.
I'll still have to give this a more proper play again before I can rate it, but I liked what I was able to play. If you plan on developing this more, I'd suggest adding x10 versions of your inventory pickups so it won't spawn so many distinct items when you die.