From Longman Dictionary of Contemporary Englishwatertightwa‧ter‧tight /ˈwɔːtətaɪt $ ˈwɒːtər-, ˈwɑː-/ adjective 1 WATERa watertight container, roof, door etc does not allow water to get in or out → airtight2 PERFECTan argument, plan etc that is watertight is made very carefully so that people cannot find any mistakes in it Lucky for him, his alibi is watertight. Unless the ban is watertight, EU laws will overturn it.
Examples from the Corpus
watertight• There will be a rolling maintenance programme of the external walls and roof to ensure they continue to be wind and watertight.• The agreement drawn up when property changes hands must be absolutely watertight.• But the Basle convention fails to offer a watertight answer to the awkward question: which waste is hazardous?• It seemed the basketwork was not as watertight as intended.• On this framework they constructed an airand watertight chamber, with the curtain wall as one of its sides.• a watertight compartment• The cameras had been stored in watertight containers.• Most wooden ships were watertight in harbour, but they all leaked when they got out to sea.• A completely watertight one was obviously impossible; and it would also be sufficiently unusual to be suspicious.• To build these structures under water, the work place is enclosed by watertight walls.• Or thaw it in cold water in its watertight wrapper for more rapid thawing. 