Good ideas. guys!
For me, the frost that melts during the day and soaks the mattress from underneath is the problem... Building the slats to keep the cushion off the vberth plywood makes perfect sense. I have used large sheets of 1/8 inch polyethylene foam under the cushion, but water still gets in.
I don't like sleeping in a wet bed.
Now I have to take apart the vberth cushions every morning to keep them dry...
Even if I don't like thick mattresses, ( I prefer thermarest for cushioning) foam helps stabilize me when the boat rocks. I bet memory foam is even better at that...I will have to try it some day. Does it absorb water?