Bluebird standard size servos are water tight. I have tested this twice.

First, I dunked my Neptune. The rudder servo was a Hitech and it was full of water. The elevator servo was a Bluebird and it was dry inside.

SEcond, just to really check, I got another Bluebird standard, hooked it up to a 2.4 receiver and battery and dropped it in a glass of water. I moved the transmitter stick for a couple of minutes. The servo was moving around because the arm was touching the side of the glass. After I got tired of wiggling the stick, I turned off the radio and checked the servo. Bone dry. I use Bluebirds in my seaplanes.