What stupid thing does Odysseus do as he and his men are sailing away, and what makes this especially stupid as they are traveling on the sea?

As they are sailing away, Odysseus tells Polyphemus his name. Consequently, Polyphemus, son of Poseidon, tells his father what happened to him, and Poseidon does all he can to divert and punish Odysseus as he continues on his journey.