I like the idea of a metroidvania which is set mostly underwater. The sprites created by your 2d artists looks nice but they lack animation. For instance there was this crab enemy on the shore which was moving but it's legs weren't moving. You run out of oxygen too fast. There are those water bubbles which you can replenish oxygen, it's a nice touch(i saw that mechanic on Banjo Kazooie)but they're not everywhere. And player character looks some kind of humanoid see creature so oxygen shouldn't be a problem in that case i think. It's hard to see through the game mainly bcs of this issue for me. The area where you speak with the granny and the captain, there are no tilemaps, it's only covered with purple sprites. I think you ran out time before finishing that area. The level design is too identical, there not much visual cues on where to go and you can get lost easily. And lastly you can't get out of conversation with the captain character, which is a game breaking bug.
I think if you fix those issues and add some more content, it could become a really good game. Best of luck on your works!