Learn Russian: Это море.

In Russian, you can usually identify a noun's gender by its last letter in the nominative (dictionary) form. The word море ends in -е, which makes it a neuter noun. As a general rule, nouns ending in -о or -е are neuter.

Is Это in the neuter form here because море is neuter?

It looks like a match, but it is actually a coincidence! When you use Это to introduce something (meaning "This is"), it always stays exactly the same, no matter the gender of the noun. You say Это море (neuter), but also Это брат (masculine) and Это сестра (feminine).

Where are the words for "is" and "the" in this sentence?

Russian does not have articles like "a" or "the". Also, the verb "to be" (is, am, are) is almost always dropped in the present tense. Because of this, Это море translates literally as "This sea", but it correctly means "This is the sea" or "This is a sea" depending on the context.