Nice game! Most everything feels polished—from the character control, to the art, to the overall aesthetic. There's a lot of functionality in this little piece, too, with sidesteps, running, inventory, several enemy types, and a boss with multiple states. It's impressive!
In it's current state, I think it's missing some metroidvania elements, though. It's linear, and as such doesn't have any backtracking or gates the player has to return to later. There are hints that some of that was planned, though, so maybe a future version will deliver on that part.
Also, I would've liked a little more explanation for things like the rocks and the healing...moss (I guess? Sorry, I don't know that much about marine life ; ), and a way to remap the mouse buttons, especially since there's no in-game need for the cursor information—I have a hard time platforming single handed (I'm more a console gamer, so keyboarding is limited to Civ and FPSs for me, haha).
Overall very good entry!