Synonyms for Watertight (same or very similar meaning)
WordNet sense 1 (of such close construction as to be impermeable):
tightWordNet sense 2 (without flaws or loopholes):
bulletproof, unassailable, unshakableWordNet sense 3 (incapable of being contested or disputed):
incontestable, incontestibleFrom the ODE community, based on WordNetadd/edit