Actually, here's what the Pokedex says about Groudon,
That said, in worst case scenario we could evacuate the areas of volcanic activities, an, like you plan, wait some number of years.
Pirates we may be, but we are cool, comfortable and covered. Best thing for a hot, buggy forest!
You would have to have strict weight regulations, and someone to check very far down on whatever supports are holding you up. With as many people and facilities as you mentioned there's no way your city will float. It might be able to be suspended over the water, but you'd have to be close to some form of land for the water not to be too deep to build supports down to the floor.
I don't consider short-sleeved midriffs very covering. Team Magma is more covered than Aqua.
See, we're not attempting to take them away from what they know. It's easier that way, if their lives get to remain the same, or at least similar, to how they are on land already.