The Atlantic Ocean lies along the eastern side of Assateague Island, with the park's boundary extending ½ mile into its waters. The ocean acts to constantly sculpt the shore by transporting sand to and from offshore areas and along the coastline itself, and by occasionally washing sediments over the island and depositing them in the bay to the west.

Many animals living on Assateague depend on the ocean as a source of food. Gulls feed on the abundant fish and shellfish, as evidenced by debris from shells dropped onto rocks, roads, and boardwalks hard surfaces. Dolphins, too, hunt among the off-shore schools of fish, as do many commercial and recreational fishermen.

The oceans around Assateague contain vast amounts of plankton. Phytoplankton produces as much as 80% of the Earth’s oxygen and serves as the first level in marine food webs. Along with zooplankton, it provides a food source for organisms ranging in size from small fish and invertebrates up to huge, filter-feeding whales.

Water temperature in the surrounding ocean fluctuates throughout the year. In winter temperatures dip to near 40 °F, while during a warm summer the water can reach into the middle to upper 70’s. As the temperature and weather conditions change, so do the animals inhabiting the offshore waters. Many species of fish, birds, marine mammals, and sea turtles migrate up and down the coast, following the most favorable conditions.