I've used a product by "Concrobium" -Mold Control. It's in a spray
bottle with a blue and light green label. I think I got it at Home
Depot. Seems to work well.
I use it periodically and I also used it the day before I painted.
(try and pick a dry few days...if there is such a thing in your area)
My belowdecks paint went well.
A trick to keep mold at bay is leave a regular light bulb on 24/7 when
your at dock. Just one 75 watt bulb keeps things drier below.
But, my attitude is these are boats. Moisture and the problems that
come with it are a small nuisance, but I don't fret too much about a
little mold or mildew, let alone small leaks through toe-rails and
stuff. I would just drive myself crazy trying to completely eliminate
it so I just try to deal with it as best I can in-between sailing.