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.
The Mediterranean Sea was the body of water completely surrounded by the Roman Empire.
Antarctica is surrounded by the Southern Ocean.
a medium size body of water surrounded by land is a sea or a huge lake.
A body of water almost completely surrounded by water is called an ocean. Oceans will often run into each other.
The Mediterranean Sea was surrounded by the Roman Empire.
A body of fresh water surrounded by land is a lake or pond.
moat.
waterfall
Lake.
Peninsula
An area of land surrounded by water is called an island.