There are ways to waterproof a servo. One include dipping the servos in plasti-dip.
Do a google search for "Waterproof Servo" You'll get tons of results including how-to videos.
Here's one article -
http://www.societyofrobots.com/actua...of_servo.shtml
You should be able to use the waterproof emaxx servos. They are standard size servos which should be a direct drop in. The horn might be different, but a simple adapter will solve that problem and probably came with your flux.
So you can either waterproof your existing servos or buy a waterproof one. It's cheaper to waterproof your existing servos, so I would recommend doing that first. If it doesn't work, you can just get the traxxas servos.
Welcome to RCM man!