I need a servo to rotate the Ping Sonar over 90 degrees. It is not simple.
These would work but still need to be enclosed with a shaft seal.
Because they are high torque and geared they will not be bothered by the drag of a shaft seal.
They may not last long if driven continuously.
Be careful; encoder might mean speed encoder, not absolute position.
If you want to stay away from seals you need to go to very expensive commercial servos.
I have thought of hydraulic actuators but they still have shaft seals to deal with.
See my discussion on the subject Rotating Shaft Seals - General Discussion - Blue Robotics Community Forums
Magnetic coupling has a problem of high pressure on a thin separator wall that may have a large area.
You may find something here Waterproof Actuators | Underwater, Subsea Actuators for ROV, UUV, AUV (unmannedsystemstechnology.com)
If you come up with a simple cheap solution please let me know.