waterproof/watertight = submersible
water-resistant = splashable
...that is my understanding too. I would tend to refer to a seal as being watertight, but to a complete technical object like a camera casing, a watch, a servo as being waterproof, with the terms overlapping. As an example, take a look at what this website says about waterproof watches:
http://www.luxist.com/2008/04/22/things ... oof-watch/
Language details aside, let's continue with waterproofing a servo.