yeah I did look into rf cameras, they have a lot going for them:
- cheap to buy
- can have lots down there and only one wire ging to the surface, use a switch to decide which one is on
- will handle long ranges much better - the analogue signals wil get noisey and won't die as obruptly as a digital one
you cat5 cable will work well for sending the rf data - simply use one of the twisted pairs, and ground the other one.
Another option is a C328 UART camera. I think theres a library for it for the arduino. No where near the resolution/framerate but has the added cool factor!
I did ponder on using rf cameras for quite a long time, but really wanted to stick to digital for future development. On refelction an rf would have given me a much greater range and just as much flexibilty (and a lot less headaches! ) so if I where you I'd go with that
One quesion - are you going to fill your bilge pumps with oil? I'm concerned about the pressure on the motor seals and thinking about it.