Very good game! I played with extra difficulty due to my cat deciding he needed a nap on the table mid game so I had to finish with the keyboard on my lap and somehow I found all the shells.
I think you nailed the movement, it felt heavy and floaty enough like you would expect a submarine to feel but not frustrating enough to be clunky, plus the it's a horror game, you don't want smooth ninja platformer movement. If I were to critique the controls I would say shooting the harpoon felt a bit unintuitive because you need to be facing the direction you're shooting, which makes perfect sense but when combined with mouse aiming it led to some missed shots. It's not that bad though, it's just a matter of getting used to it.
I also think you nailed the atmosphere, I was freaking out at every fish, octopus and crab that showed up. The black and white colors looked great for the dark deep sea theme, especially with the use of dithering for shading. If I were to critique something I'd say the shell was overly pixelated while everything else was not, it felt out of place, especially when side by side with the stones and the submarine with way higher pixel count and dither shading on them. Oh, and also the drop down for the theme picker wasn't themed correctly.