Learn Russian: Мы обедали в морском ресторане.

When simply stating the general fact that an action took place in the past, Russian usually uses the imperfective aspect. If the speaker wanted to emphasize that they completely finished their lunch (for example, as a completed step before going to a museum), they would use the perfective пообедали.

Why is there no word for "had" in the Russian sentence?

In English we say "to have lunch", but Russian uses dedicated verbs for meals rather than combining a noun with "to have". You use обедать (to eat lunch), завтракать (to eat breakfast), and ужинать (to eat dinner). Saying something like имели обед would sound very unnatural.

The English translation says "seafood", but doesn't морском just mean "sea"?

Yes, literally this translates to "sea restaurant". While the noun for seafood is морепродукты, you don't typically use it as an adjective to describe a place. In Russian, a seafood restaurant is naturally called a морской ресторан (sea restaurant) or a рыбный ресторан (fish restaurant).

Why do the words морском ресторане have these specific endings?