If you are using servo motors for any underwater activity, although these hitec waterproof servos are super nice, you don't have to spend 50 dollars per servo. Here's how to waterproof servos yourself in a few steps. I havent tested this at super low depths, nor have I done extensive tests for long periods of time. However, I have made little robot fish that are totally fine after many swim sessions using this method.
Currently, I'm making a swimming robotic snake like this one using this method.
I've read that people have successfully tried and used the condom method (wrap your servo motor in a condom, basically) but my condom broke : ) when I tried it.
Step 1: Materials
Auto marine sealant (DAP works well but another silicon sealant will also work)
o-ring, like this one : EPDM O-Ring, AS568A Dash Number 007
Step 2: Seal the seams
There are two seams that run around the servo motor body. One is about 1 cm from the top, and the other is right along the bottom, where the screws are. Using silicon marine sealant seal with a thick layer all around the seams. Make sure to also put silicon around the spot where the wires enter the motor, as well as a lot around the 4 screws on the bottom of the servo motor.
(Note: In this photo, I am using a bathtub & tile silicon sealant but it adhered way worse than the silicon auto/marine sealant)
Step 3: Hang them to dry
Let them dry for as long as the glue requires.
Step 4: Unscrew the servo horn
Once your silicon has dried, unscrew the servo horn, (or the plastic round part that comes on the servo).
Put it to the side.
Step 5: Lube up the o-ring and slip it on
I'm using SuperLube synthetic grease in the photos here but I prefer marine grease. But any dielectric grease will do. Make sure if you are using silicon o-rings, however, not to use a silicon lubricant.
Lube up the o-ring and the servo motor shaft, and put the o-ring on.
Step 6: Put back on the servo prop or horn
Put back on the servo arm of your choice. Make sure you line up the center if you aren't using the round arm (You can gently twist it back and forth to see its degree of rotation and what is the center).
Screw it back in, and you have a watertight servo motor!
Now, make a Robo-Snake!