Really cool! I'm amazed at how much you managed to do in just 72 hours. I could get lost exploring underwater and listening to the chill soundtrack. As the others pointed out, I do wish the swimming speed could have been faster, though it did make the underwater section feel so huge because of the time it took to navigate it.
One little nitpick is that the animation for the character's head seemed to be quite buggy underwater. Also, when I first went under, I had no idea where to go and didn't find the first collectible for nearly two minutes. Spawning closer to the first collectible followed by seeing one not too far off in the distance could go a long way.
Great work!