User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4902593-20130225075602/@comment-3974711-20130225134130

That's... a tricky question. Poseidon is overall the god of the sea, and any hydrokinetic powers come from being that. Based on what Percy can do, it's clear he controls water and moisture, as those things can be seen in the sea. Ice? Debatable.