Often dolphins slap their tails on top of the water to frightening hiding fish into moving so they can find food. Dolphins will also sometimes slap their tails as a sign of annoyance or as a warning to predators or other dolphins.
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Humpback whales have been known to slap their fins on the the surface of the water to stun their prey or as a form of communication to other whales in the vicinity. Anything other than that, I do not know and you will have to research on a deeper level.