I have learned to use a product called "Corrosion X".
Just take the bottom off your servo and dip it into the liquid and let it dry. We all use this for both boats AND airplanes flown on floats.
You also could invest in waterproof servos. Not water resistant but true waterPROOF servos. They do work.....have tried them..
[link]http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTKL0&P=SM[/link]
Lenny