Really cool game! The level of polish is unreal, congratulations ^^!
The graphics, sound design and level design are really good ^^! I really enjoyed the music changing the deeper you go, that's a nice touch. The combat was fun and challenging, but the harpoon mecanic was a little bit strange. I understood the harpoon -> knife combo, but when I didn't one-shot the enemies with it, I stoped using it because it gave the enemies free hits.
The progression if very slow, unfortunately ^^'. I would have enjoyed it better if the first ugrades where cheaper, and the late upgrades were more expensive. And, I must say, not a lot of upgrades seemed really good because they come with a debuff that sounds scary (like losing 1hp/s ? With so few healing? I don't know what is "1hp" because the health bar doesn't show any numbers, but that sounds bad!).
The camera is waaaay to high! Since the way to go is always "down", having the character at mid-screen makes it so 50% of the screen is nearly useless... I was often hit by fishes because they saw me before I saw them ^^'.
But hey, I'm focusing here on the 2-3 little things that bothered me because everything else in the game is absolutely stunning and solid! You did a really great job with that game, and it's incredibly polished. Congratulations!