In the first chapter of Percy Jackson and the Olympians: The Lighting Thief, Percy unwittingly uses the water to grab Nancy Bobofit:
I don't remember touching her, but the next thing I knew, Nancy was sitting on her butt in the fountain, screaming, "Percy pushed me!"
Mrs. Dodds materialized next to us.
Some of the kids were whispering: "Did you see —"
"—like it grabbed her—"
I didn't know what they were talking about. All I knew was that I was in trouble again.
Percy Jackson and the Olympians: The Lighting Thief, chapter 1: "I Accidentally Vaporize My Pre-algebra Teacher"
Why didn't the Mist obscure the water grabbing Nancy Bobofit?