@Hespia:
when the strawhat crew went to the mangrove, they explained that the special resin produced by the trees are maintained by the special atmosphere around the mangrove, right? and when the bubble drift too far (too high up) it will pop.
if that's the case, then how are the coated ships of the whitebeard maintained? and the coated highways of fisherman island? shouldn't they pop as soon as they're out of the mangrove?
We can only asume that there are other rules for the bubbles/coating when they are under water. You know that the coating of the ships burst as soon as it touchs the surface as seen with Whitebeards ship and the one Zorro sliced up.
As for the coated highways i would explain it in this way that Fishmen island is surrounded by the roots of the mangroves. I wouldnt even be surprissed if the air in FI and the substance for the coating/bubbles also comes from the roots of the mangroves.
EDIT: @Gladiator yeah that is what is between my sentences^^