Both terms refer to a creature living in water or a thing operating there. The first difference is that aquatic always an adjective, while marine can either be a noun or an adjective. If it is a noun, it refers to a soldier serving on a ship or at a naval installation. Aquatic refers to a being growing or living in water; it describes the properties and characteristics of an animal or a plant in water. It also means some kind of activity in water, any kind of water sport. Also in biology, it can refer only to freshwater or together with its environment. Sea and ocean are different terms to distinguish it from the rest; in this case, it is called marine.
