No need for "water proof" IMO. Buy the Hitech servos, and if you'd like to you can waterproof 'em yourself. Just go to you youtube and search
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the best thing you can do is mod them.
ballons with orings/silicone is one way.
another good way is to take the servo bottom cover off, and fill the whole area with dielectric grease. dielectric grease is non condusive, and water cant get in to a fillded cavity. use orings and/or silicont around the screws, case seams, and drive gear.
i only did this in two stock traxxasssss servos. didnt pose any notable issues right up till the crap ass servos poop out.
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I cut/pasted that from another forum - but it gives you an idea.