Riptide — George Martin

The second Tiny and Big Shorty novel. They camp on The Outer Banks of North Carolina. When a Riptide traps two people, they meet some of the locals and two female twins. A map from Salty's Emporium shows the location of Blackbeard's Treasure, but the barrier islands have shifted position since then. They must deal with sharks, The Coast Guard and the villainous Hawk in a quest for treasure.