So as you know Percy is the son of the sea god so he is awesome with water.
So he was swimming and he went deep into the water and was swimming by some seaweed, and he saw something but he was wondering what it was.
He stops really quietly and that something he saw comes stop straight in front of him. It was a mermaid!
They got higher and Percy heard Annabeth calling him. So he said bye to the mermaid and went to Annabeth and went to the camp fire.