The Adventures of Hotsy Totsy by Clive Cussler (paperback)
Casey and Lacey Nicefolk seem like two regular, all-American kids. But these twins have a secret: a magical box that can turn any small object life-size! So they dream up a plan to turn a toy boat into the life-size Hotsy Totsy and enter a national boat race. With their dog, Floopy, along for the ride, the twins zip up to San Francisco, ready for excitement. But this is no ordinary boat, and this is no ordinary race. Casey and Lacey must work together in a nautical adventure that turns into a risky race to the finish line!