The short answer: They’re not!
Cranberries grow on low lying vines on the floor of the bog. For a more extensive breakdown. Read more about how cranberries grow here.
Why do so many people think cranberries grow in water? When fall rolls around and it’s time to harvest the cranberry the grows begin a process called wet harvest.
During wet harvest the cranberry bogs are flooded with up to 18 inches of water and the cranberries float to the surface! The cranberry contains a few tiny air pockets that cause them to float. Learn more about how cranberries are harvested here.