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Deals damage and has a critical hit rate one stage higher than normal. Assuming there are no other critical hit modifiers in play, this move has a 1/8 chance to get a critical hit instead of a 1/16 chance.
Crabhammer is a semi-exclusive move for the Kingler and Crawdaunt lines. It's a more powerful alternative to Waterfall, with a slightly higher Base Power and increased ability to land a critical hit, at the cost of lower accuracy. Because it's semi-exclusive, the only users are those already mentioned. Most of the time, however, it is outclassed by Waterfall's consistency, thanks to the flawless accuracy, flinch chance, and basically same Base Power. Most battlers will prefer not to risk missing at a critical moment, especially with the formidable damage output that these Pokemon are already capable of; though if you want the extra power or critical hits, then there's nothing stopping you. It should be noted that Kingler is incompatible with Waterfall, so has to use Crabhammer regardless.
Waterfall is a less powerful move with perfect accuracy and a chance to flinch the opponent.