His reflexes prove too slow, and Weasel finds himself falling. He falls for 40 feet, wind whistling in his ears the whole way, and then his body impacts against a dark, stagnant body of water (9 points of nonlethal damage). He quickly realizes that his situation is still dire, and he must try to swim, lest he sink and be unable to breath.
Swim check.
Meanwhile, Talon sees the halfling drop through the now-apparent pit trap.