Note that the title is managing, not preventing! Dampness is an inevitable fact of living above a large body of water out in the open so water can come in from any direction. Add to that the moisture in the air from evaporation of bilge water and from your breath, there are many sources of water and not all can be prevented.
This article lists many of the water sources but focuses on the most difficult to manage, air borne moisture.
Stopping water from above
Rain water and sea spray can enter from deck fittings, window and hatch seals, the anchor hauser pipe, the mast, shroud chain plates, cockpit drains, etc. Almost all of these are preventable with a bit of sealant and maintenance.
Stopping water from below
Sources include the prop shaft, through hulls, transducers, osmosis, and cracks or holes in the hull. All can be sealed/repaired, but these usually require the boat to be out of the water.
Managing air borne moisture
Unless you stop breathing, completely seal your bilges, and only sail off the sahara coast, this one can not be prevented; it has to be controlled and directed.
Warm damp air rises; it is less dense than dry air. When this air hits a cold surface, the moisture condenses on that surface, forming drops of water. We have some control over which surfaces are coldest and can often direct that water down to the bilge. An efficient automatic bilge pump can remove nearly all of that water.
Managing which surfaces are cold, and therefore where the water forms needs a combination of ventilation and insulation to prevent moisture condensing where you dont want it and have cold surfaces in places where you can direct the water drops into the bilge; its important to think about how the air will flow in the space to ensure the damp air passes over the cold surface. If you can get this right, you can create a dehumidifier using the hull or cabin top as a condenser.
On Lady Cindy, I have four spaces: the forward V berth, the heads, the saloon, and the engine compartment. Of these, I am only concerned about managing airborne moisture in the V berth and the saloon.
The V berth was the priority as I had made it my sleeping cabin, and it had no insulation apart from a couple of side panels; the roof was plain fibreglass so got cold, especially at night. My breath overnight resulted in a dripping cabin roof and wet hull surface in the morning, soaking my bedding.
After lowering the bed by six inches to gain headroom and widen the bed, the old side panels no longer fitted. New ones were made from ply, and the fronts covered with foam backed headlining and backs with foil covered bubble wrap. The hull behind the panel was covered with the same foil covered bubblewrap. The insulation is designed and marketed to insulate garage doors. There is now an air gap between the hull and the panel where any water vapour can condense and run down to the bilge, very little does as although not sealed, little air can flow into the space.
The bubble wrap insulation has also been placed against the hull at the back of all lockers in the V berth, heads and saloon to prevent moisture condensing there, each space still has a cold uninsulated locker where most of the moisture can condense and drain to the bilge. This has been partially successful. I am having to double the insulation on some lockers to improve things.
The roof was more complex; I covered it with the same foam backed headlining, but I had to ensure damp air did not gather here but instead flowed forward to the chain and sail locker at the bow to condense on its cold surfaces or through to the saloon and managed there. Which direction the air goes depends on wind direction outside as this determines the direction of air flow through the cabin. There are four external air vents on the boat: Anchor hawse pipe, V berth roof, heads roof, saloon roof, and cockpit door. There is a gap above a panel separating the V berth from the bow anchor locker for air to flow between those areas. I also installed a small 12v fan so air can be moved even if there is no wind outside.
What about windows?
As I am not prepared to replace all of these with double glazed units, I have to accept water drops will form here and drip down. I wipe them daily and ensure there is nothing below them that could suffer water damage. Keeping the boat warm and ventilated is the only solution, but I do have a small 12v dehumidifier which helps a little.
Is my boat now dry?
No, that would be impossible, but my bedding is dry, stuff in my lockers no longer feel damp, and black mould is no longer an issue in my living areas. I would call that a success.