Answer: watertight

AnswerCrossword Clue
WATERTIGHTLeakproof
WATERTIGHTLike a perfect alibi
WATERTIGHTLike a Navy seal
watertightClosely sealed, fastened, or fitted so that no water enters or passes through
watertightso well-sealed as not to admit water or let it escape
watertight(of an argument or account) Unable to be disputed or questioned