[sanddrag]

I've never done this but I had an idea.

You might consider keeping the servos inside the watertight enclosure/body, and running a music-wire linkage through the enclosure to the outside. Where the linkage passes through, I'd design a sealed grease box that basically has an O ring on each side where the linkage rod passes through, and the box is full of a very thick grease (maybe like wheel bearing grease). This grease box would not have to be very large. I'd think this would keep out the water.

EDIT: This is what I was thinking of http://en.wikipedia.org/wiki/Stuffing_box