The sound isn't caused by the claws snapping together, but rather a jet of water which is shot at 60 mph. Due to the, um, "underwateriness" of the action scene, a low pressure bubble is formed and the sound is created when the bubble collapses. And the sound is so powerful it stuns the opponent into a somewhat retarded stupor, leaving Mr. Pistol to reap the rewards and tuck into a lovely comatose crustacean snack.
The sheer force of the bubble collapse means it reaches temperatures up to 18,000 degrees Fahrenheit, hotter than the surface of the sun.