On our recent Seascape cruise of Oct. 29, 2023, we embarked at Terminal C.
The lower decks used the west door (door closest to Miami), and the higher decks used the east door (closest to the ocean). We were on Deck 5, but I don't remember how high the lower decks went, but yes, there are two doors, and they are clearly marked. This was nowhere on our documents, but I read about it here on Cruise Critic prior to our sailing. Thanks again, Cruise Critic!
By the way, although we embarked at Terminal C, we disembarked at Terminal B, so we had a rather long walk to get back to the Terminal C parking lot after the cruise.
Happy cruising!