I really like the idea of using sonar to navigate, I think it's well done in this game. The game is difficult enough to make me want to come back and keep playing. I encountered some interesting situations where my sonar had made it through a slim crack, and I crashed not being careful enough.
I think the reward system might be a little slow. Or, I'm just complete crap at playing it. But, in all, it's a fun little game.
Thank you, and good luck.