Isn't "These warm and shallow seas." the correct translation for this sentence? Translating "These are warm and shallow seas." would be "Ezek tengerek melegek es sekelyek.", right?
"These warm and shallow seas" seems like a half sentence to me, waiting for something like "are full of life". Your Hungarian sentence is missing the article from before tengerek "Ezek a tengerek melegek és sekélyek." I'd translate that as "These seas are warm and shallow".
In addition to this, I would like to humbly point that if the adjective is positioned before the noun it must be in singular. On the other hand, if it is positioned after, it should be in plural (of course if the noun is in plural).
After reading your reply, and looking at it with a fresh mind after a break, I understand now. Thanks!
Not always. No one talks about the Pacific Sea, or the Mediterranean Ocean.