A Piper PA28-235 can have 84 gallons of fuel in 4 tanks, so leaving
them half empty, for instance in a humid maritime environment,
overnight where the temperature drops sufficiently to cause the
moisture to condense out of the 40 gallons of air contained in them,
will result in enough water in the fuel system to interfere with
operation of the aircraft's power plant.

At 40 degrees Celsius, 1 cubic meter of saturated air contains roughly
40 grams

That's about one and a third ounces. Remember, if all of the water comes out
of the air, it settles to the bottom, and is either drained or used by the
engine. It would take a lot of such cycles before enough water would be in the
tank to matter. As someone else already said, full tanks to prevent dangerous
condensation is another aviation myth.