Underwater communications can be done with with audio sound waves. Sonar is what you'll need to look at.
Basically you could use piezoelectric transceivers (cheap under water wafer, that converts electrical signals to sound, and vise-versa). You would want to use a signal within the range of human hearing, but a 5k Hz signal is a good place to start. if you keep the transmission at a digital format (one's and zeros) then communication electronics would need to be changed from radio frequencies to sonar frequencies. you would need to have your remote control unit, configured for sonar and the transceiver for that unit will need to be in the water; along with your sub. Sound waves, at a given signal strength, will travel farther in water; than in the air (deals with the density of the sound media, i.e. water has a higher density than air). Fresh of salt water will not make any measurable difference in signal performance. In simplest terms, the piezoelectric transceiver replaces you antenna. You can get piezoelectric kits for cheap. There are also experimental kits available which will give you some insights. IMPORTANT NOTE: the transceiver is waterproof BUT the electronic connections are NOT.
