Sonic data transmission? Answered
This is just an idle thought, but I was reading a couple of ROV builds, and the one thing they all have in common is a tether - it's a fact of the world that very little in the way of radio signal can penetrate water.
Could you send control signals to an ROV via sound or ultrasound? Could beeps and clicks carry enough data, far enough, and clearly enough, to control an ROV?
Even more fun, how much data could come back? Sensor readings? Images?
Just throwing it out there for you to toy with, but it would be fun to have genuinely-remote control for this class of robot.