Lazar, Ralph. Total mayhem #4

"Dash and his friends Rob and Greta are used to unusual things happening at their school, Swedhump Elementary. But even they pay attention when their invisible principal, Mrs. Rosebank, announces that they have a special guest: Ms. Aqualine, the owner of the new Aqualina Water Park. Most elementary students don't get to spend time on waterslides during the school day. But soon Dash, Rob, and Greta are slipping and sliding, right into a mystery! It turns out there's more to this water park than splishing and splashing. There's something very evil going on, and instead of soaking it up, Dash and his friends are going to catch a wave and ride it to victory in the war of the waterslides."--Back cover

720L