My own version of the water plane is like a crystal estructure an alternating succession of spheres A and B. One is a water sphere, and the other is a void with space and a sun, also some floating islands. But the plane is two overlay layers. If you swim or diving, you will find only water, but there is sun light and air from the empty spaces from the other layer. This allows surfaces with the same hidrostatic pressure. Next to the "water plane" there is a "reef demiplane" where you can air-breather sea animals, or even coast civilitations. Of course tritons and other sentient creatures would rather these "reef demiplanes" to be totally secret to avoid attacks by pirates and those things.
My story about the origin of the fire plane is really this elemental fire is different, because it is not ordinary matery, but more like "plasma" or dark matery. In the begining it was an infernal plane to punish sinners, or infidels who prayed other deities. After primal war some fire gods and other powers claimed as their own domains, and later some zones were "remodeled" to allow visitors for planar trade. Then the goal is the fire plane is working like a "cosmic firewall" to stop or avoid possible planar invasions. Of course there are lots of "elemental realms" where mortals are wellcome to become potential wordshippers of certain fire deities. Other reason is because the fire plane is the best place to build "magitek energy plants". This energy is "gathered" and sent to the eco-friendly factories in Mechanus. Some regions are more like a purgatory, where sinner souls suffer penance before be purified and allowed to enter into the celestial planes.
The main reason because industrial revolution didn't started is because the most advanced industry is in Mechanus. And the taboo because no-magic machines can be possesed and controlled by hostile spirits.