Place a watch on the hook inside of a pressure tester. Close the lid of the tester. Pressurize the tester.
Now here is the important part ... if the watch leaks, it will fill with pressurized air. If it does not leak, it will not fill with anything.
Lower the hook and watch now into the water of the tester. Once it is submerged, slowly release pressure from the tester via it's release valve.
If the watch has a leak, the pressurized air from inside of the watch will now come streaming out, making many many bubbles.. You must draw the watch out of the water with the hook. No water has gone into the watch because it was streaming out pressurized air.