Something that is watertight does not allow water to pass through it, for example because it is tightly sealed. The flask is completely watertight, even when laid on its side
without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument"
A watertight case, argument, or agreement is one that has been so carefully put together that nobody will be able to find a fault in it. The police had a watertight case. They even got his fingerprints from that glass cabinet