WordNet (r) 3.0 (2006):

belly flop
    n 1: a dive in which the abdomen bears the main force of impact
         with the water [syn: belly flop, belly flopper, belly
         whop, belly whopper]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "belly flop":
   belly buster, belly whopper, cannonball, crash dive, dive, drop,
   fall, gainer, header, jackknife, nose dive, parachute jump, pitch,
   plunge, pounce, power dive, running dive, sky dive,
   stationary dive, stoop, swan dive, swoop