Sealed against the wind and rain.
So tightly made that water cannot enter or escape.
(figurative) So devised or planned as to be impossible to defeat, evade or nullify.
‘a watertight contract; a watertight regulation’;
having no leaks
not allowing water to pass in or out
without flaws or loopholes;
‘an ironclad contract’; ‘a watertight alibi’; ‘a bulletproof argument’;