ORIGINAL:
If you know that you will only be in 20-30 foot of water could the sub be wired to a surface float/boat that hauled the reciever? that way your antenna is always above surface sending instructions down the chord..not sure if that has any advantages over having it hooked to the boat you are already in?
I think that what foodstick is saying is to run the reciever on the surface with 20-30 feet of wires goint to control the servos. The antenna length would not change. My concern would be any voltage loss with 20-30 feet of wire.