It is a good point, but there are far more hospitable places in the Solar system - Europa, Titan, Saturn's rings, without much of Sun's radiation, more heat and water than Mars can ever dream of. Also a bunch of metals to mine.
Europa has liquid water, but the average surface temperature is still colder than the worst ever recorded on Earth. The speculative oceans you seek would be encased under miles of solid ice, just like Titan's. Europa is likewise catastrophically radioactive.
These places are all extremely small, extremely far away and extremely deadly. They make Mars seem pleasant.
Good luck storing water and other supplies though. I foresee water piracy in the floating cities.