What is a proper noun for sea?

There isn't a single, universally accepted proper noun for "sea." However, some examples of specific seas with proper nouns include:

* The Mediterranean Sea

* The Black Sea

* The Red Sea

* The Baltic Sea

* The North Sea

You could also use Ocean as a proper noun, referring to one of the major water bodies:

* The Pacific Ocean

* The Atlantic Ocean

* The Indian Ocean

* The Arctic Ocean

* The Southern Ocean

The best way to use a proper noun for "sea" depends on the specific context and the sea you are referring to.