1. not allowing water to enter or pass through
2. having no flaws or weaknesses; impossible to dispute or defeat
Usage: figurative

Examples

• The submarine’s hull must be completely watertight.
• Make sure the container is watertight before storing it underwater.
• The boat’s watertight compartments prevented it from sinking.
• Her alibi was watertight – no one could question it.
• The lawyer presented a watertight case to the jury.
• We need a watertight agreement that covers all possibilities.
• The prosecution’s argument wasn’t watertight enough to convince the judge.