I have had several planes with standard miscellaneous servos mounted under the wing and never a problem. Water exposure to the servo is momentary and it blows off as soon as you take off or pull any Gs. I suppose water could wick uphill with capillary action, but it has to get thru the greasy gearset first before it can hurt the electronics.

Now a servo buried in a float is entirely different. I'd dunk one of those in wax before using it that way.