You might do a search for the topics revolving around bunk condensation
. While, yes, typically they have to do with cold weather
, & moisture forming in & under bunks due to temp differentials caused by cool air. They have a lot of good; suggestions, methods, designs, & products, which aid in assisting with increasing air flow in, around, & underneath of bunks.
Which... I'd imagine would help a bit when it's warm also. It certainly seemed to when I lived aboard anyway.
As bunking on top of a piece of insulation
, mounted on an impermeable layer leaves no where for the H2O to go, or fresh air to get in.
I can't say that I'm the biggest advocate of a camping mattress on a boat, as typically they're just a thin piece of closed cell foam. Maybe with a few ridges or bit of texturizing. But in my experience, they don't have much at all, built in, to handle moisture management. So whatever's between you & the mattress gets soaked oh so much quicker than with a better system.