NASA has a beautiful video visualizing ocean currents around the world. Far from being static bodies of water, the oceans are in a state of constant flux:
Watch surface currents circulate in this high-resolution, 3D model of the Earth’s oceans. Driven by wind and other forces, currents on the ocean surface cover our planet. Some span hundreds to thousands of miles across vast ocean basins in well-defined flows. Others are confined to particular regions and form slow-moving, circular pools. Seen from space, the circulating waters offer a study in both chaos and order.