For Robotics, servos are a lot more useful than motors but for going underwater they both are useless so need to be "water-proofed" so in this I'ble I will show you how to do it with the most efficient method.'
Step 1: Water-Electric Info
The hobby servo in the last 5 or so years has become extremely popular among robotics builders. They are cheap and easy to use, but there is one issue with them - they aren't waterproof. Whatever people claim, servos are only water-resistant. This means that splashing water on them is fine, but full water immersion is like drowning cute kittens (very bad). This tutorial will list several popular ways to waterproof servos, then I will go into the best method (the SoR method) towards the end of the tutorial.
Servos are Only Water Resistant
Taking apart a standard servo you will quickly notice thin rubber rings around the edge, as shown at the bottom.These rings are there for near water-tight seals. Your main concern for water entering your servo is actually at the output shaft. Sealing the output shaft is always the most difficult part when waterproofing motors, and people do crazy things like magnetic coupling, etc. But as a hobbyist, using the most cherished of robot actuators (the servo, cough), we will have to do this low budget. But note, when I say low budget, I also mean very reliable . . .
Electrical Resistance of Water
The very first question you should be asking yourself is what type of water your robot will operate in, as it will make a huge difference in your design.
Pure water is a near perfect electrical insulator - meaning that it conducts electricity very very poorly. If you were to drop your computer into a tank of pure water, your computer will still work! So why is water normally bad for electronics? This is because in water there are impurities called ions, such as sodium or chlorine or whatever, that transfer electrons.