water goes to the lowest point since it weighs more than fuel. then if it's not drained properly it gets sucked into the engine at the start of the flight. would be odd to discover water contamination after a long-haul flight.
I kind of assumed the FOHE was something that used external air, it wasn't water in the oil or the fuel. you descend through humid and coolish air ie not summer heat, water condenses and freezes as it's sucked through something from the pressure differential, or from actual icing conditions. but I could be wrong.
>water goes to the lowest point since it weighs more than fuel
While that's true, it takes time, and it won't get enough of the water out of the fuel[0]. Also, the aircraft must not be disturbed for the water to settle out. Operation demands for aircraft don't allow that. Even if they did, some water will remain in the fuel.
While the engines are operating, fuel is constantly circulating which disperses any water within the fuel. Airport fuel supplies at major airports are usually tightly controlled, but it's still be possible for water to get in.
>I kind of assumed the FOHE was something that used external air,
The FOHE is a Heat Exchanger, sort of like the radiator in your car, but instead of engine coolant/air the working fluids are Engine Oil/Jet Feul. Both the fuel and oil systems are closed systems, with one exception. As fuel is consumed in flight, the volume of that fuel must be displaced by outside air. Fuel tank to atmospheric pressure must never exceed some low differential pressure. This is one source of moisture ingress that must be dealt with, and can cause water to accumulate in tanks if left unchecked.
>you descend through humid and coolish air ie not summer heat,
Are you aware that the air temperature at altitude 35,000 ft is around -55C?
Icing in the fuel lines may be caused by fluid flowing through an orfice (pressure differential). All that is needed is water in the fuel.