Learn Hardcore Latin: Nauta aequor spectat. - The sailor watches the open sea.

Nauta is in the nominative singular, which is the case normally used for the subject of a sentence. It also matches spectat, which is third person singular: the sailor looks / is looking.

So in Nauta aequor spectat, nauta is the sailor doing the action.

Why does nauta end in -a if it means a male sailor?

Because nauta is a first-declension noun, and first-declension nominative singular forms often end in -a.

In Latin, a noun’s ending shows its declension pattern, not always its natural sex. A few first-declension nouns referring to male people are masculine, such as:

nauta = sailor
poeta = poet
agricola = farmer

So nauta looks like many feminine first-declension nouns, but it is actually masculine.

What case is aequor here?

Here aequor is accusative singular, the direct object of spectat.

The sailor is doing the seeing, and aequor is the thing being seen. So:

nauta = subject
aequor = direct object
spectat = verb