I remember back in the day (dating myself here) quite a few ROV systems utilized RC for control, I can remember numerous times piloting a Benthos Mini-Rover MKII and every time someone keyed a radio the ROV would spaz out. Just a word of advice, stay away from RC control. The RC was typically transmitted via coax which would also have the video feed inside the tether wire bundle.
Thanks Craig, I guess the new RC gear with either sbus or ibus uses some type of serial interface
to transmit the signal from the bus to the servos, EPS etc. The speed of the serial communications is rather fast, which would limit the length or depth of the tether.