I've been using Aeroplate from Aerotrend. I am doing a project boat for another site and it doesn't have a radio box. The first time out I flipped the boat getting the reciever wet and it wouldn't work. I had already put Aeroplate in the servos so they were Okay. I opened the reciever, sprayed some in being careful not to get any on the connections, put it together and it worked! It's about the price of a cheap servo for a bottle and will do a lot of equipment. I wish that I had bought some years ago, it would have saved me the price of a lot of servos and recievers that I've fried!
Some people over on Jim's R/C Boat Dock have been using Corrosion X in their gas boats with no radio boxes except for the reciever and have had no problems.
Most LHS's can get Aeroplate if they sell Aerotrend products, the guys at mine didn't even know what it was!