Atlantia, hypothetical water world (without Kevin Costner)

With all the worlds we've seen on our way through the Spiral, it seems at least possible that there is a water world out there somewhere. I remember from when I was doing the Marleybone book quest, that one of the books said that the Marleyboneians (?) believed that all the worlds of the Spiral had once been part of the same planet. If you believe that, then the oceans must be out there somewhere.
Now, I'll admit a world comprised mostly of water does seem kinda out there even to me, but somethings holding the other worlds together and providing them with air. So that something could keep all the water from slipping away into the void as well.
I'm not sure what the driving force would be but depending on where it falls in the time line, it may or may not involve the big M. All I know is that something is driving the undersea people to invade the land.
The way I imagine it, the commons area would a semi-large island. with all the merchants and what not, and the 'streets' would be mostly underwater. Fighting the enemy underwater would allow for 3 dimensional combat like Autumn suggested. The first 'street' could be mostly above water with a little underwater and the main quest line would have to do with you helping build some magical diving equipment that would allow you to go underwater longer.
I'm not sure about the natural inhabitants of the world, all I can think of is otters and turtles. But a good idea might be to have races from all over the spiral fill the holes in the population. Ignoring pirates completely, wouldn't a mander dressed up as a sailor with a bandana and everything be incredibly cute.