Nancy Drew Clue Book: Pool Party Puzzler

Snooty Deidre Shannon writes in her blog "Dishing with Deidre" that since eight is half of sixteen, she's entitled to a Super Sweet Half-Sixteen party this year! And since the Shannons have a pool, she's decided to make mermaids her theme. Nancy, Bess, and George are invited to the big bash, where all party guests must dress in under-the-sea-themed costumes.