Great Point Peter. The good part is that I'll be close to it controlling it. Distance wise it wont go out of sight...We hope!
If possible, I would have a floating RF antenna connected via cable to the ROV. Other than a cable directly to the control unit, that would be the simplest way to communicate the the ROV. There have been a few previous threads dealing with underwater communications and submersible ROVs (like this one
). The fact of the matter is that wireless communication underwater is significantly harder to do reliably than in air. Not only are most of the RF bands greatly attenuated, but most optical frequencies as well. Even ultrasound, which at first glance would be more effective
underwater has potential problems, including varying propagation speed potentially causing a signal to interfere with itself and with thermoclines
(sharp boundaries between two layers of water with significantly different temperatures.