I got very Master Blaster vibes from this game, so that was cool - a very good use of the Grow and Shrink theme. The atmosphere was great too, the black and white and only occasional sound effect gave the whole world a very unsettling vibe, and the intro did a good job of setting the treacherousness of the trench up.
Some things I struggled with
1. The speed and pace of play. I get that you're underwater and the speed of movement did a really good job of selling that atmosphere, but it just took forever to do anything or get anywhere, especially if you died and had to start over. I would have enjoyed a better balance between realism and speed.
2. Once you get the submarine, it's kind of hard to know what exactly the little guy is used for. I sent him outside the submarine to get some fish sometimes, but his gameplay didn't feel super differentiated aside from not having a weapon, and I would usually just sprint to the fish and then hit escape to go back to the sub. Giving him his own particular niche aside from being able to fit through one-tile gaps would help with this.
I admittedly didn't get super far in the time that I played (by the fourth time I died, I didn't have it in me to navigate through the cave system again), so I don't have a lot more feedback. I think if you were trying to sell a very oppressive, alien atmosphere that makes you feel super vulnerable, then you definitely succeeded at that.