I like the sonar idea, and the enemies legit surprised me when I first saw them! the game creates fun tense moments, and I was legit excited about different power ups ("faster sonar cooldown?! YES PLEASE")
if you'd told me it was set in outer space instead of under water, I'd've believed you. more visuals and water sounds could help sell the "underwater" theme... but who am I to talk: your graphics are better than mine :P