4 -5 feet is not too deep, could you use plastic fiberoptic cable and a surface buoy? And a modulated 5mW laser may be better than the IR signal, especially range and power. Use a phototransistor for detecting the laser pulses... I've done similar around the lab for transmitting Morse Code between Arduinos and the phototransistor works even in ambient light - but you may wish to consider a optical bandpass filter anyway.
Just a thought... but it does make things more complex.
Ray