great game, super addictive. the upgrades climb well and the fish are cool. it's satisfying killing cthulu as well.
here are some issues:
first off, the last fuel upgrade should be infinite. it's just about impossible to reach the bottom without skipping everything, given that you need to maneuver around the bosses to kill them. even with every upgrade, the difficulty of reaching the bottom AND killing the last boss is too much for me, and i'm a completionist, so that bothers me. also, i found some edges i can squeeze through (far right side, top) and swim fairly far without encountering anything. also, with the upgraded sonar, cthulu is a joke, you just go above him and he doesn't respond, just takes his punishment.
other than that, it's a great game and i think it deserves a sequel.
here are some suggestions for said sequel:
choice of weapons that are meant to target specific fish (as a real fisher would change bait) so you start with a harpoon, good against squid, bad against schools of fish, machine gun - low dam high ROF, lasers - good against cthulu, and of course the classic depth charges. Cthulu should be the last boss and be like a wall (he was too small here in my opinion) so you get the satisfaction of defeating a gigantic boss and there's no area to get around him, cuz he takes up half the screen.
oh, and maybe some replay value? like an absurd amount of money needed to customize your ship's appearance or something, or create-a-fish. after finishing the game, i see no need to visit again, really.
again, great game and i look forward to seeing more from you.