water condenses in fuel in the manner stated in above posts. However 747 and 767(and I must assume other A/C) have a small pickup at each fuel tank boost pump. It uses venturi effect to continually suck up small amounts of fuel and or water from the very bottom of the tank and deposits it at the inlet of the pump and gets used.
Therefore you will never find water in the tank after an A/C has landed but you may find 50-100 mils after the A/C has been on the ground for a day or so. Obviously the amount of water varies with the conditions.