watertight

from 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}]

from 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]

from 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