Wtf this looks amazing!! I wasn't expecting the 2.5D from your thumbnail, it looks really good! The placement of the foreground elements, the blur, the extremely well animated enemies, everything comes together really nicely !
I think the most noticeable thing that feels like it's missing is sound. The monsters don't make any sound and there's no sound when you get hit. Which, I imagine in a submarine you probably don't hear a whole lot, but some hull damage at least maybe?
The enemies were super aggressive and I got stuck on them for a while. If chomper fish sees you he doesn't let up. I also didn't realize that the 1st item I picked up was the propeller, at first I threw myself at the enemies a dozen times before not having any way past, wound up at the propeller, had no idea what it was or did, and then coincidentally looked at the manual and saw that there was a button that moved fast. I thought I had noob bridge'd myself until I died and could no longer do it, yet I tried throwing myself at enemies a few more times because I assumed it was working and I just wasn't noticing the difference. I think just a simple text box that tells you what you picked up and what key to press to use it would help clear things up.
They could also be pretty unfair. Once while I was around 290 m after getting the drill, a chomper came up out of nowhere at full speed, instakilled me, and flew off into the night. It was a tragedy :(
The drill was kind of difficult to control, I'd maybe make the hitbox on it more forgiving since it's easy to get stuck between rocks. When I first got it I rammed straight into the rocks next to it and couldn't figure out why nothing was happening
I gotta say though the mood descending to 300m is reeeal good, it's very moody. The drill room too, very cool look. This is a very lovely game!