Why is the phrase "under the sea" constructed the way it is? Shouldn't it be "in the sea?" Does "sea" refer to the surface of the ocean in this case? Or is it just one of those quirks in English?
Under is used to refer to something below a surface:
Macmillan dictionary has:
e. below the surface of water:
The ducks kept diving under the water to catch fish.
He was the first person to claim there was oil under the North Sea. She jumped in the pool and went under.