The Collaborative International Dictionary of English v.0.48:
Water-tight \Wa"ter-tight`\, a. So tight as to retain, or not to admit, water; not leaky. [1913 Webster]WordNet (r) 3.0 (2006):
watertight adj 1: not allowing water to pass in or out 2: without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument" [syn: unassailable, unshakable, watertight, bulletproof]