...and what happens when you test your watch and its not waterproof?
The same damage is done
In order to use these testers you need to take the dial and movement out, reassemble the case and then test. It will then show that it the case is water proof (or not). You then take the case out of the tester and you then put it all back together again ready for use.
They would be fun to play about with but they are of little practical use to anyone who does not mod or service.