Some oceans contain areas which have their own names, such as the Sargasso Sea, in the Atlantic. But normally this is applied to regions marked by landforms. or adjacent to them.
Flows of water within large bodies are called currents.
(see the related question below)
A body of water such as you describe would be the ocean itself.
An island is a body of land completely surrounded by water.
A body of water almost completely surrounded by water is called an ocean. Oceans will often run into each other.
The Mediterranean Sea was the body of water completely surrounded by the Roman Empire.
Antarctica is surrounded by the Southern Ocean.
An island. Or if your really picky, a floating rock.
The Mediterranean Sea was surrounded by the Roman Empire.
A lake is a body of water surrounded by land. Lakes are typically enclosed bodies of water that are not directly connected to the ocean.
moat.
waterfall
Lake.
Peninsula