Yeah, I spent some time trying to find the balance between not so wide is trivial and not so narrow is annoying, I wanted to make a point of having to move really slowly and methodically to get through the more narrow caves but I agree it's annoying in retrospective lmao, specially since you can't rotate the submarine itself. I can see a future with caves that you can traverse at full speed if you know what you're doing...
Also the damage at low speed was left mostly because otherwise you could just avoid damage by going really slow, but that definitely could use some polishing and the reason is pure speculation. I remember making the hitbox of the submarine smaller that the submarine sprite to compensate though