Be able to healed by water
When Percy Jackson is in Camp Half-Blood he is role-playing a battle against another team as a training exercise. Just as he is about to get to the final goal he gets into dual with Annabeth Chase. She beats him down and he is injured. A voice (which turns out to be his father, Poseidon) tells him to go to the water, where he uses the water's power to heal himself.
Going along with the story of the movie, it certainly seems that this is the first time he does this as Percy himself is surprised.
But, I am sure growing up he would have hurt himself before and just taking a bath or even going for a swim would have healed any of his injuries big or small. Surely he would have realized this long before going to Camp Half Blood?