Over the Sea's Edge — Jane Louise Curry

Dave finds a curious silver medallion in a cave by the Ohio River -- and then finds he has changed places with Dewi, a sixteenth-century boy in medieval Wales. When he travels across the sea with explorers searching for the legendary cities of the New World, Dave hopes to find a way back to his own time and place. Meanwhile, Dewi is adjusting to life in the modern world. As the boys journey through time, caught up in a world where prophecies come true and past becomes the future, they find that they may be better suited to the other's time than for their own.