Hi Eliot,
Sure. So may i present my system?
The RC signals are transmitted to the buoy. An sbus receiver inside the buoy sends the sbus signals through the tether. An sbus decoder inside the ROV generates the PWM signals for the ESCs. Quite simple. The AV video signal takes the opposite direction and is sent via transmitter inside the buoy to the FPV googles.
This equipment is light, quite inexpensive, reliable and KISS (keep it simple, stupid).
A laptop or a ground station would be a hindrance.
That would be the best possibility, but i´ve got no knowledge how to create or edit firmware. 
By the way: the rc signals can send to a pixhawk via sbus. Tha´s no problem.
Greetings
Robert