This was a fun game! The water movement felt really nice, and I enjoyed the art quite a bit. The swimming animations was SUPER realistic, I couldn't believe it was pixel art. I liked how the camera could pan a little bit ahead in the direction your mouse was pointing too, to look ahead for danger. I enjoyed just swimming around looking for the armor pieces and upgrades quite a lot.
But I felt like the harpoon was inconsistent. Sometimes it would launch out and stick, and sometimes it would instantly retract before leaving the character sprite. I had to kind of cheese the flashlight area by swimming around and luring the big fish into the open area because I couldn't do it the right way. As soon as I got the gun, my enjoyment level skyrocketed. Especially once I learned that bullets (like fish) can go through walls.
The angler fish boss was a really fun fight as well once I understood that the light that rotates around hurts you (and also that I could just hold the fire button instead of mashing it). I thought it was a really smart mechanic to have the player regain health by doing damage, it actually encouraged me to get into fights throughout the game.
Great work!