“Go on, then, son of Neptune!” Polybotes taunted. “Let me see your power! Does water do your bidding? Does it heal you? But I am born to oppose Neptune.”
The giant thrust his hand under the water. As the torrent passed through his fingers it turned dark green. He flung some at Percy, who instinctively deflected it with his will. The liquid splattered the ground in front of him. With a nasty hiss, the grass withered and smoked.
“My touch turns water to poison,” Polybotes said. “Let’s see what it does to your blood!”

The Son of Neptune, Chapter L