wrote

But the small amount of water that enters each time the surroundings
cool, stays there and accumulates. Over a 4 or 6 month layup?

Not necessarily. The surroundings have to cool differerentially or to the
extent that there is water on every surface. Even then, the water doesn't
necessarily fall off the surface and into the fuel. I was in my boat nearly
every day working on it last winter and I never saw conditions that made me
think that the tank top would have been covered with water drops. I also
kept my tanks drawn down most of the winter because I was installing my new
fuel system. My boat was in a shed that warmed up a lot with a very damp
floor so there was lots of temperature differential.

When in turmoil, when in doubt, always try a little perspective:

The "pumping action" of temperature changes is orders of magnitude less than
the pumping due to fuel sloshing around with boat motion.
The temperature differentials in summer with cool water and hot sun are much
greater than for a boat hauled out in the winter.
The cold air of winter is generally drier. Condensation happens because air
cools and can't hold as much moisture. Much of that process has already
happend by the time winter air gets into your tank.

If filling your tanks is really vital in winter, big time problems in summer
is what we would be discussing here.

It still a good, but not vital, idea to minimize fuel surface area contact
with air over long periods of storage. One way to do this is to fill the
tanks into the vents. Another is to empty the tanks. I can't do the latter
so I do the former. Not doing anything is unlikely to lead to problems so
severe they can't be dealt with with something like Startron.

My boat was laid up for six years near Detroit by the former owner with a
partially filled fuel tank and I had to deal with it after I bought it. The
fuel turned to partly to jelly that wouldn't go through the filters and
there was a mega clean up but there was no water in the bottom of the tank.
There was no real alge growth either. The fuel just evaporated off ligher
elements, oxidized, and aged.

--
Roger Long