Will they be parasailing in tandem or separate? I'd have them take the camera themselves onto the boat for pictures. You don't need a water camera, but it's safer if you do have one. One father/daughter on our boat who parasailed in tandem took a video camera up with them. I took a water camera with me, as did my parents who went with me. If you have two cameras, they could ask another passenger to take pictures of them parasailing while they take pictures from the air.
It will be very difficult for you to see them from shore unless they are wearing something very bright so that you can notice it from far away.