The Collaborative International Dictionary of English v.0.48:

leakproof \leak"proof`\ adj.
   having no leaks.

   Syn: watertight.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

leakproof
    adj 1: not subject to leaks

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "leakproof":
   airtight, ballproof, bombproof, bulletproof, burglarproof,
   corrosionproof, dampproof, dripproof, fire-resisting, fireproof,
   flameproof, floodproof, foolproof, hermetic, holeproof,
   impervious to, moistureproof, noiseproof, proof, proof against,
   punctureproof, rainproof, raintight, resistant, rustproof,
   seepproof, shatterproof, shellproof, soundproof, stormproof,
   stormtight, tight, waterproof, watertight, weatherproof