Bribe a Kid

Picking out a likely accomplice, you kneel down next to the pool.  "Hey, can you do me a favor?" you ask.

The boy in the pool, who has a thin mustache and a shell on a cord around his neck, looks up at you.  "Perhaps," he says.  He has a Spanish accent.  "What do you need?"

"Can you splash that old man over there?"

"I can," he says.  "But-"

"Just splash him," you say.  "I'll get you a soda or something."

"All right," the boy says.  He heads over to the edge of the pool, looks around furtively, and then you see a distinct gigantic fish fin come up and splash water all over the old man!

"I didn't come to the pool to get wet!" the old man jumps up and runs away.

"Hey!  That kid is a merman!" you yell.

Sit in the lawn chair.