Winter can be an especially dangerous time for fuel contamination by water. Frequent inversions and high humidity can combine for condensation forming in the fuel tanks. To prevent that condensation from forming in the first place ensure that the aircraft are put away with full fuel tanks. The condensation enters the tanks through the tank vents in humid conditions and if the tank is kept full there will not be room for that humid air to enter the tank.

The water vapor that enters the tank through the vents will sublimate into ice crystals which will be suspended in the fuel.  Identifying fuel contamination is more difficult when there is suspended ice crystals as opposed to water as the ice crystals do not settle to the bottom of the sample cup.  To recognize the ice crystals look for an added sheen to the fuel sample when viewed through the side of the cup.

Last winter we had especially humid conditions and prolonged periods of no activity because of the awful weather.  One of our aircraft that had been sitting for a couple of weeks with half tanks had almost a gallon of water and ice crystals in each tank.  That would be enough water to stop any engine likely soon after takeoff.

The other risk associated with water contamination of fuel in the winter is frozen fuel lines. Not much you can do to fix that problem in fight. All you can do is try to remove as much water during the preflight as possible.

Moving the aircraft into a heated hangar might be the best option for melting the suspended ice crystals in the fuel so they can be easily removed.  We also add isopropyl alcohol (rubbing alcohol) to the fuel as it tends to disperse the water unlike ethylene glycol (gas line antifreeze) which surrounds the water and creates goobers of water.  Those goobers can pass easily through automotive fuel systems but can cause significant blockages in aircraft fuel systems. No matter what every attempt must be made to remove the contamination prior to flight and you should consult your aircraft and engine manufacturer for their recommendations under such conditions.