aequor

Aequor, aequoris, n., third declension neuter, from aequus, "level". Literally "the flat thing", and it can be a plain of land, which is how Lucretius sometimes uses it. In practice it is a poet's word for the sea, chosen when the water is being pictured as a smooth expanse rather than as mare, the ordinary noun for the object.

That choice tells you a good deal about a text. Virgil reaches for aequor constantly; Caesar, describing the same Channel, barely at all. Two mechanical points. The nominative and accusative singular are both aequor, so the case has to come from the syntax. And the plural aequora is the commonest form of the word, but Latin poets pluralise the sea as a matter of course, so aequora very often means one sea, not several.