/ˈwɔːtərtaɪt/
water·tight
BrE /ˈwɔːtətaɪt/
NAmE /ˈwɔːtərtaɪt/,
/ˈwɑːtərtaɪt/
that does not allow water to get in or out
a watertight container
(of an excuse, a plan, an argument, etc.)
carefully prepared so that it contains no mistakes, faults or weaknesses
a watertight alibi
The case has to be made watertight.