Pond
No, a lagoon is not a pond. A pond is a small body of water completely surrounded by land. A lagoon is a small body of water that is partially cut off from a larger body of water.
A pond or a lake or a puddle.
It is a bog.
An island is a body of land completely surrounded by water.
A body of water such as you describe would be the ocean itself.
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.
A peninsula (which means "almost an island") is a relatively narrow body of land that sticks out into a body of water, and is surrounded on all sides but one by the water. (The other side is attached to a larger body of land.) A bay, on the other hand, is a body of water largely surrounded by land. Only a small part connects to the rest of the water (e.g., the ocean.)
Antarctica is surrounded by the Southern Ocean.
The term firth can be used to describe a small body of water extending into a landform (estuary is also used in this context).If this body of water is surrounded by mountainous land, the term fiord or fjord can be used if one wishes to be more precise.
An island. Or if your really picky, a floating rock.
The Mediterranean Sea was surrounded by the Roman Empire.