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Deals damage and has a high critical hit rate.
Crabhammer is an exclusive move for the Kingler line. While Surf is more reliable and has 5 more Base Power, Crabhammer has a high critical hit rate, which means Kingler is going to get a critical hit with this move almost always. However, keep in mind that Kingler has a low 50 Special, so it will absolutely need the critical hit to deal any damage. It will be able to overcome Amnesia and Growth boosts, though.
Surf is a more powerful move with perfect accuracy but normal critical hit rate.