If you are thinking about gettin cheap knock off waterproof servos.....dont!!!
I'm getting ready for a race and found 3 bad ones so far. None were submerged as far as I can remember and every one I opened had Corrosion after only 1 or 2 weekends of racing.
Even if they're labeled "waterproof", no matter the brand, I always take them apart and put a little silicone around the rubber grommet where the wires come in and around the top cover base gasket. Running a $15 Injora 7kg sub micro servo mounted directly to a gas outboard engine for throttle. No problems.
+1 on what scott said as well as spraying the with Corrison X as soon as you get them...take it totally apart and spray liberally...let dry a day or so and silicon grommet and reassemble...........havent lost a servo in a long time doig it this way.