and about The FI.
It`s definitely one smaller bubble inside a huge bubble.(you can see it even better on the smaller bubble on top..it also is a smaller bubble surrounded by a bigger bubble).
I guess in between them is water, because so the harbor appears to float and with water in it it`s actually useable.
I thinks it`s some kind of entrance as we saw in Skypia, where fishman try to see whos entering the island.
After that comes the inner bubble, which seems to have air inside it, because we see some smoke here and there rising in the bubble(the smaller on top) and some kind of clouds(in the big one).
But they also use like on Shabondy some small bubbles as we can see.
The city itself seems to be made out of corals and seems to have many hidden and dark places in between them.
In the smaller bubble on top it seems like a castle is located..could be the place where the crew gets to know the mermaid-princess and Jinbe.
That we have real actinism visible: mhhh, i think a huge bioluminescence "sun" could be located a bit over the island..maybe from a giant fish the fishman control.