I know I'm going to come off as a real jerk here. But maybe she should stick to swimming in a pool. If they don't like what's in the lake, don't go in the lake. *shrug*
I would have been incredibly bored when I was a kid if my parents removed all of the snakes and spiders, and "undesirables" from wherever I happened to be playing.
To answer your question: I can't think of anything you could put down there to scare them out. There's a reason why they're there. Probably because it's dark and secluded. Maybe you can open it up some so more light gets in. Maybe trim the bushes around it or something.
But consider this: When they're under the dock, you know where the are. Scare them out and they could end up anywhere. If it's a cottonmouth for instance, I'd much rather have it under a dock than underneath my bare foot while I'm walking on shore.