After Clarisse tried to push Percy into a toilet bowl, he felt a pull in his stomach and water burst out of all the toilets, flooding the bathroom and drenching Clarisse and Annabeth.
As they are walking back to cabin 11, Percy doesn't realize that he cause the bathroom incident until Annabeth clears it up for him.
During capture the flag Clarrisse and her friends hurt Percy, but after hitting the water, Percy was rejuvinated and broke Clarrisse's spear.
After breaking Clarrisse's spear and winning capture the flag, A HellHound was summoned. After the HellHound was killed Percy was pushed in the water and a trident appeared over his head confirming his father to be Poseidon.