Consider that empty tanks do not mysteriously fill with water when no one is looking.
Consider that fuel when refined goes through high temperature 'cracking' or/and high temp. distillation which 'drives off' any water.
Consider that tanks containing fuel oil do become *saturated* with water, including water in a separated phase at the bottom of the tank ... all that water entering through the vent which is openly connected to the humid/wet atmosphere.

So, if youve followed the (chemical equilibrium) logic up to this point that dehydrated fuel picks up its water from the humid atmosphere, it would seem prudent for not having WATER in your fuel:
• to remove the fuel during long term layup (dispose of it in your oil burner tank at home),
• to only keep on board the amount of fuel that you will use in short order and maybe with some extra for 'emergencies',
• and only buy your fuel that is 'fresh' (still relatively dehydrated) either from a high turnover fuel source or a 'truck stop'.

If you always 'top off', you WILL ultimately have fuel that is ALWAYS fully saturated with water (either in emulsified/molecular form or in 'free' form at the tank bottom due to 'gravimetric settling') from the atmosphere. Why do this ??????