Learn Russian: Отдых на море — это хорошо.

In Russian, we use the preposition на (on/at) with море when talking about being at the seaside or having a beach holiday. We usually only use в (in) if someone or something is physically submerged inside the water.

What case is море in here?

It is in the prepositional case because it tells us location, answering the question "where?". The word море (sea) is a neuter noun that already ends in -е, so its spelling doesn't actually change here; the dictionary form is also море.

Why is there a dash and the word это in the middle of the sentence?

Russian does not use a word for "is" in the present tense. Instead, when equating one idea with another (like saying "X is Y"), Russian often uses a dash followed by это (this is) to connect them. It translates literally as "A holiday at the sea, this is good."

Why do we use the adverb хорошо instead of the adjective хороший?

When we make a general statement or give an evaluation of an activity or situation using это, Russian grammar requires the short adverb form ending in -о (хорошо). You are describing the overall concept of the holiday, rather than attaching an adjective directly to a specific noun.