@Urouge:
There are quite a few things both within the main Fishman Island bubble and in the surrounding area as well. All of it appears to be connected by bubble roads, which makes travel between any of the areas convenient enough to skip. I'm pretty nervous about Oda forgetting the 10k meters of pressure thing at some point, to be totally honest.
As far as we could picture it for ourselves, we can assume that the main huge bubble(FI) is connected to many other smaller bubbles which are for themselves also "small underwater islands".
The bubble we see here:
http://read.mangashare.com/One-Piece/chapter-607/page011.html
"on top" of FI i think was a bit misleading.
In my opinion its not on top of the FI, instead of just being further away in the backround, so that it appears to be on top of it.(you see also the main island is floating..so the island behind it could just be floating a bit higher afterall) It
s just another one(when not even the Sea-Forest bubble) of the smaller bubbles, which are connected to the main bubble via those roads.
The Sea Forest also has to be such a smaller bubble outside of the main bubble, because Luffy and the rest arent using bubble-helmets. We already knew from the scene with Robin(where she is riding the bus) that the fishman developed a good infrastructure..which suggest that those roads have to be a bit longer, which makes it plausible that some smaller bubble-islands could be further away.(but still in the range of the Sunny Tree Eve) Because of such an advanced infrastructure, i think that the waterpressure shouldn
t be of any problem, Urouge, because they sure are keeping those roads up in a good condition.
Also we saw that those bubbles arent bursting that easily down there..(as we saw while Zoro,Sanji and Luffy were fighting the Kraken–even when this wasn
t exactly 10.000m under the sea there) and streching them to such roads shouldn`t take away much of their stability.
For the fishman-district, we can say that it has to be most likely on the ground somewhere under the main bubble.
(indicators are:
-Noah, the sunken ship..because where else could such a big ship lie, when not on the bottom of the seafloor (ironic enough that the NFP using a sunken human ship as their headquarter, because iam not sure if rly fishman should have build it underwater, where it is..well useless
-the scene with VdD, because he had to use his bubble-helmet, which shows that it has to be underwater and not inside a bubble
-that the NFP are now accroatching the island, which somehow makes it look like for me as if they weren`t before on the island (but sure controlling it already))