Tool handle dip. Wipe the servo case with alcohol, let dry, then paint on the dip covering all seams. Make sure you cover the wires completely by untwisting them then painting each wire for an inch from the case, you may retwist after painting. Grease an o-ring and place it on the output shaft, tightening the arm down only until the servo resists. Should keep water out for brief periods and several feet under.