Sigh… if you insist dints.
I've been dreaming lately (yes I dream one piece... it's why I'm here after all) about a deep sea bioluminescent giant sponge reef. Sea sponges are filter feeders that live by filtering out food particles floating freely in the water. The giant tunnel that makes up the connection between the first and second halves of the grandline have to be nutrient rich, considering it's the only water passage between the two hemispheres on this side of the planet. The sponges could thrive in such an environment to reach a magnitude similar to the harukiman mangroves.
And that's the other important aspect here. Mangroves work by having their root systems span from above water to the soil far beneath. This allows for easier gas transfer to their roots so that they don't asphyxiate underwater. So, the harukiman mangroves should be constantly bringing breathable air down to their roots while removing waste gasses from the same. These are the same gasses that create the bubbles in Shabondy. My impression is that the Fishmen could simply tap into the gas exchange of these root systems. Their scale, as shown last chapter, has an enormity sufficient to provide for an entire city's air supply.
Okay, follow me here. If we have giant sponges with an air supply and a water pressure resistant barrier that can be applied over anything, why not a sponge style apartment complex? Coat the outer holes of the sponge, and drain the inside. You'll have thousands of soft, cushioning, intersecting rooms. You would have thousands of small coated barriers instead of one big vulnerable bubble. The outside of the sponge colony would still be covered with living polyps while you inhabit its skeletal interior. It'd be both beautiful and weird enough to be something Oda would think up. Here are a few pics that I think could be an inspiration for something like this:
Hmmm….Aha........
Ye I'm playing dumb.