Indigenous Americans’ encounter with the world began at the water’s edge. Initial encounters with Europeans happened on beaches, on islands, and on the water itself. As foreign powers began to colonize the Americas, saltwater fringes would form some of the most profitable and contested regions. This fact, which scholars have only recently started to examine with care, goes against common assumptions about where “borderlands” and “frontiers” are supposed to take shape. Looking to the continent’s margins reveals a distinct category of contested spaces that did not work by the same rules as terrestrial ones. From the fifteenth to the eighteenth century, two particular American coastal regions faced economic, political, and cultural changes that were all connected to the underlying ecological dynamism of shorelines.