How could you be a child of Poseidon?

You were nothing more than a horribly lucky idiot, in Andrew's eyes. You had arrived at camp barely three days ago, taken under Luke's wing, and presumably sucked at everything besides canoeing.

...so he used you for Capture the Flag. Clarisse hated your guts, and Andrew used that to his advantage rather quickly. He sent you out to "guard" the river, which was code for "you're going to get your ass kicked while I score the flag".

Perfect idea, crafted by the perfect son of Athena.

But of course, you just had to get claimed by Poseidon when Clarisse shoved you in the river. Boom, water bends to your will, you can breathe underwater.... the weird glow-y trident thing....

Andrew wasn't impressed.