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]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "watertight":
   airtight, ballproof, bold, bombproof, bulletproof, burglarproof,
   close, compact, corrosionproof, dampproof, dripproof, dustproof,
   dusttight, fast, fire-resisting, fireproof, firm, fit for sea,
   flameproof, floodproof, foolproof, gasproof, gastight, hermetic,
   hermetically sealed, holeproof, impervious to, leakproof,
   lightproof, lighttight, moistureproof, noiseproof, oilproof,
   oiltight, proof, proof against, punctureproof, rainproof,
   raintight, resistant, rustproof, sea-kindly, sealed, seaworthy,
   seepproof, shatterproof, shellproof, shut fast, smokeproof,
   smoketight, snug, soundproof, staunch, stiff, stormproof,
   stormtight, tender, tight, water-repellant, waterproof, weatherly,
   weatherproof, windproof, windtight