• waterproof; hermetical; cannot be refuted
• a watertight container or door does not allow water to get into it or past it because it is tightly sealed.
• if something such as a guarantee or plan is watertight, it has been put together very carefully, so that it deals with everything which it should deal with, and nobody can criticize it or argue about it.
• chamber in a ship that is sealed against water
• proven plan, tested plan