What is the difference between an ocean and a sea? What is the difference between an ocean and a sea? The term sea is used to describe the global, interconnected water body covering 70% of the Earth's surface, whereas oceans refer to the individual basins within this global sea. The term ocean is used in the Northern Hemisphere, and sea is used in the Southern Hemisphere, so the difference results from geographical location. Oceans tend to be composed primarily of salt water, while seas tend to be composed of a near even mixture of brackish and fresh water. Seas are generally smaller than an ocean and tend to be at least partly enclosed by land, such as large, inland, saline water bodies. An ocean is a vast body of water encompassing 70% of the Earth, whereas a sea refers specifically to a large, inland, saline body of water.