Great and original concept! There isn't much to the game right now, but I did have fun trying to get a high score (I especially enjoyed trying to get two ships to hit each other). I think this base idea has a lot of potential and would be very interested in seeing it expanded upon.
Probably my biggest suggestion would be to make the game harder over time (maybe you already do this, but I couldn't feel it). Right now, it feels like once you've got the hang of it, you can pretty much go indefinitely, which did make it start to feel boring when I got to around 3000 points. Even if you did something simple like add more and more ships and have ships start firing at you even when you're not using sonar, I think it would give the game a lot more replay value.
I also encountered a bug where ships could sometimes suicide with their own bullets, which often happened at the edge of the screen, but it wasn't a big deal.
Overall, nice work!