Very good game. Core gameplay and progression play with each other well and keep engaged, not to mention that the core gameplay is really novel. I would suggest two minor adjustments - two little cherries on an already delicious cake, that by no means really needs them. Both are related to sonar:
1) I dont's see any visual representation of the sonar's source, and as far as I can tell, the sonar always keeps its local rotation to the sub static when not used. So whenever I start using it, it appears on a seemnigly random side of the sub. Together with a pretty slow (initial? I suspect it might appear in the upgrades somehwere down the road) rotation speed - which is fine by itself - it gets a tat annoying at times.
2) Compared to all the other art in the game, the sonar animation just feels draft-y or placeholder-y. As with 1), doesn't really hinder the experience.
To conclude - brilliant job!