What Greek hero drowned when Poseidon struck the rock he was clinging to with his trident?

The Greek hero who drowned when Poseidon struck the rock he was clinging to is Polydeuces, also known as Pollux. He was one of the twin sons of Zeus and Leda, and a member of the Argonauts. In the myth, Poseidon, angered by Polydeuces' arrogance, caused the rock to sink, leading to his demise.