I've been getting condensation under the fwd mattress when I sleep on the boat. The wood under the mattress gets pretty wet and I started lifting the mattress up during the day to let it dry. I tried using a flooring materiel between the wood and the mattress but while it kept the wood dry, it trapped the moisture between the sheet and the mattress so the mattress stayed wet.
I then purchased 14 ft (it's about 4ft wide) of hyper vent. This is a breathable "paper" with a stiff plastic weave about 3/4 inch thick underneath the paper. I cut this to fit the v-berth and tried it out. The mattress and the paper stay dry but the moisture still collects on the wood. I don't think I have enough air flow to move it away during the night.
So, I saw a video of a cat which had small circles cut all over the boards over the water tank and a small computer fan in the front panel to pull air out of that space under the bed. I think I'll try that next and hopefully that will fix it completely.