A small one is called an island and a large one is called a continent.
An area of land completely surrounded by water is called an island.
Islands
It is an island.
A lake?
This could be a number of things. A pond, a lake, an inland sea, etc.
A large area of land which is completely surrounded by water is a continent. A small area of land which is completely surrounded by water is an island.
An area of land almost completely surrounded by water is called a peninsula. A peninsula is a narrow strip of land that extends out into a body of water and is connected to the mainland on one side.
A freshwater lake.
An island is a body of land completely surrounded by water.
A country that is completely surrounded by other countries is described as landlocked.
That depends on whether the land is surrounded by water or something else. If it is surrounded by water, it is an island. If it is surrounded by water but is sufficiently large, it may be called an island continent (an example is Australia). If it is a land area completely surrounded by high mountains, rivers will flow to the land that is surrounded instead of to the ocean in that area. That would make the land an endorrheic basin.
An island is surrounded by all sides with the oceans.