Jerry has the ultimate solution, but there is a lot you can do to keep the
moisture under control. Main thing is to reduce - close to zero as you can - standing water inside the boat. For us this means drying out the bilge with a 'turkey baster' every few days the thru the winter. We have rain water coming down the inside of the keel-stepped spar.
Knowing that there will be vapor present, have lots of air flow via cracking the hatches and aeolian vents, like the Beckson models. Some folks like having a bit of heat, like a low-amp rod heater inside just to help create some air circulation from the rising of warmer air currents.
Open up ALL covers over spaces where stagnant air could sit against the hull. We have taken the cushions home every cold season, November - April , for 20 years.
Also, since any dust or dirt will attract moisture and that provides a fertile field for spoors, every other year we clean up the inside hull surfaces with some bleach/cleanser and wipe dry with rags.
The result is that when the hatch is rolled open, there is zero "boat smell".
While some of the preventative maintenance takes occasional work, it's way better than wrinkling your nose every single darned time you come aboard all year long. Not to mention the pained look on the face of guests going out for a sail with you...
For clean air inside ideas, I recommend Peggy Hall's definitive book on eliminating boat odors.
