The theory of water in the fuel tanks as I was taught it a long time ago.
Fuel Storage systems "breathe" In doing so the fuel absorbs moisture from the atmosphere. When an aircraft climbs to high altitude and the air temperature falls, the fuel cools. This causes the moisture in the fuel to precipitate in the aircraft tank. Being heavier than the fuel, the water sinks to the bottom where it gathers at the sump. Draining the sump during the transit or at the end of an overnight stay removes the water from the tank.
I remember the VC10 fuel quantity indication system being particularly badly affected by water in the tanks. Once I was called out to a "fuel discrepency" and asked the engineer who fueled the aircraft if he had done a water drain check. "Yes" he said "and it was absolutely clear." When he had gone I repeated the check and also got a beautifully clear sample. But when I checked the S.G. the hydrometer jumped straight back out again. The sample was pure, crystal-clear water. We drained twenty gallons out of that tank before we got to pure fuel!
If there's fuel in the tank then there's water in there too. Always.
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Through difficulties to the cinema