Gotta have something to protect the surface water from the solar wind once it is there. Gravity needs to be strong enough and the temperature needs to be in the right range, too. Having a reasonably thick atmosphere can help.

E.g. deep craters on the moon that aren't exposed to the solar wind have ice - http://www.nrl.navy.mil/clementine2/

http://en.wikipedia....rial_liquid_water

IOW, God's will. ;-)

Cheers,
Scott.