I know that "under the sea" means someone is under the sea surface, which is equivalent to being submerged under water. If that's the case, are there any differences between "in the sea" and "under the sea"?
For example, a person is scuba diving. What is the difference between:
He is scuba diving under the sea
and
He is scuba diving in the sea
Another example:
The fish is in the sea
and
The fish is under the sea
One more example:
The submarine is under the sea
and
The submarine is in the sea