If the wetness is occurring from sleeping in the bunk it is due to the moisture sweated from the body. The problem comes up again and again in forums
Moisture travels from warm regions of a structure to cooler ones (anyone who has been involved with the design of big freezers and cool stores in the meat and food
processing industries will be familiar with that). So the moisture sweated from the body is driven from the warm part of the bed
towards the cold bunk top which forms a barrier to it progressing further and so it "puddles" (I am avoiding the misleading use of "condensation").
There are 4 solutions that I know of -
1. Ventilate the bunk top into the space below. This generally, in my observation, requires the likes of replacing the bunk top with slats, just drilling big and lots of holes is not enough. I, personally do not like this for modern boats as the space under bunks is normally not well ventilated so by doing this one is just transferring the dampness problem into the storage
space below. I have seen examples where this has been done though in custom built boats using slats for the bunk top and ventilating the space under the bunk up between the side lining and the hull
to ventilate out of slats into the cabin space - the ventilation gets driven by convection and works well.
2. Raise the mattress on a grid or whatever so that there is good airflow under it so the moisture driven down through the mattress is carried away. From what I have seen quite a lot of free air space is needed to provide adequate ventilation. Also, need to consider that if the fiddles on the side of the bunk are not high enough the mattress will not be restrained when the boat heels - not a problem on cats or for smooth water
mono sailors though. If access to the storage
space under the bunk is through hatches in the bunk top then this method can be inconvenient for access.
3. Use a normal thick household type mattress which is made to go on a hard base. It should be thick enough and open enough in structure to self ventilate.
4. Provide a barrier between the body and the mattress so that moisture cannot travel down through it - this is the method we use. This can be done by using the plastic/rubber materials used in hospitals/child beds/etc to prevent wetting from urine, blood, etc BUT all the materials of that type I have tried raise the humidity in the bed
to what can be for some an uncomfortable level even if light bedding is used over. They are also "crinkly" when one moves.
Because we needed to keep our bunk heights down (due to the way it turns out from sole height, desired bunk width, and the shape in the side of the hull
the fiddles on our bunks are too low to allow us to lift
the mattress to ventilate underneath properly as in 2 above. Also, access to the storage space under the bunks is through hatches in the bunk top. After trying a number of materials we found that a very high quality, very soft and quite expensive "vinyl" (don't actually know what plastic it is) fabric
used for upholstering furniture worked perfectly - no humidity (don't know why) and no crinkling. Is just cut to shape, laid on the mattress and a thin under-blanket placed between it and the bottom bed sheet. I was led to using this material by a race
boat owner who had all the mattresses/settees upholstered using this type of material for ease of cleaning
and he reported no dampness underneath.
It should be noted that heating
the cabin does not cure the problem. In fact it may make it worse if the space under the bunk is not also heated as otherwise the temperature gradient between the top of the mattress and the cooler bunk top will likely be increased providing more driving power of the moisture from the sleeper towards it.
Sorry if that sounds a bit "school mam'ish"
but is something I have had quite a serious look at so just passing on what I have found - hope it is of some use.