I don't like foams too much because they tend to release VOCs. But I reckon that a boat might be more humid than a regular home, so it might be tricky to get other materials to work well except wool, horsehair or kapok.
Perhaps you could find a Hästens shop near you, walk there and try one of their 8 cm horsehair toppers (TM8) without a bed. Don't buy it. It's a good product but really insanely overpriced. It will give you a good idea of how a thin custom wool mattress will feel, though. These will compress a bit more than foam, but they also breathe more which might be an advantage during warm weather.
Are you Flemish or Dutch? If so, there should be a few shops close to you.
And yes, boats are generally more humid than most homes - especially when left alone, they get extremely humid.
And yes, I am Dutch, I guess you could tell by my username :) I'll have to check out a Hastens sometime.
Else, if you are a back or tummy sleeper and it's squishy, you should try to switch to something more firm.
If what I suggested is too radical, try natural latex, which is quite firm.
A good entry point to the (pretty lacking) sleep ergonomics literature is:
Thanks a ton!
My girlfriend and I both find it very comfortable, and all the guests I've had also like it.
What do you think? Are there any hidden risks here that I'm not seeing, or reasons to evaluate the mattress on factors other than comfort? E.x. potential health risks (back problems???)
Thanks!