|MadSci Network: Astronomy|
There is some water in Jupiter's clouds, but recent results from Galileo have shown that this amount may be far less than previously believed (check out http://www.jpl.nasa.gov /galileo/status970605.html ). Currently, it looks like the Galileo probe entered in a very dry hole in the clouds and that water is more abundant elsewhere, possibly to the point of having thunderstorms and rain 50 miles below the cloud tops. More investigations are needed before a conclusion can be reached. Assuming that the clouds where the Galileo space probe entered were not typical of the Jovian atmosphere and that water is 100 times more abundant in most of the atmosphere (as stated in the article in the link above), Jupiter's clouds could be your source of water.
Your city would have to float in the atmosphere as there is no solid
surface on Jupiter. You could perhaps float near the level where there is
water and filter water droplets from the atmosphere as your floating city
moved around. Another alternative also exists, however. The moons of
Jupiter are very rich in ices. In particular, the Galilean satellites
Europa and Ganymede have thick water ice crusts. Europa is the closer of
the two to Jupiter and may have a salty water ocean below a few (up to
~100) kilometers of water ice.
If I were you, I would probably put a city in orbit around Jupiter and use the moons, particularly Europa, as a source of water. You could even put your city on Europa itself where ample water is available and would not have to be brought in from elsewhere.
Note: The pressures and temperatures deep within Jupiter's atmosphere are quite high and would crush any city if it should fall too deeply into the cloud layers. Also because Jupiter is so massive, its gravitational pull is very strong and humans would find it very difficult to move around. Further, Jupiter's magnetic field is very strong and any city will have to be carefully shielded from the high levels of radiation that result (since the magnetic field traps charged particles from the Sun). Putting the city within the clouds of Jupiter or even in orbit around Jupiter may not be technologically feasible. It would be more prudent and much easier to bury a city beneath a moon of Jupiter where it would be more protected from the radiation and would not be subject to the high gravity, pressure, and temperature.
Try the links in the MadSci Library for more information on Astronomy.