Sounds good to me. 12 cm is not very thick, so I assume it's quite firm yet comfortable enough to side-sleep?
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.
It's actually not firm at all! Very squishy. You do sink in slightly. You wouldn't be worried about back issues with something made entirely of soft foam? FWIW it's very comfortable night after night and I don't run into any back pain, but I'm still young, so maybe that'll take time to manifest itself? Back issues are my main worry.
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.
If you side sleep, I don't think you'll run into too much trouble. To be sure, lay on your side and ask someone to check your spine is straight. Your hips should sink in a bit, but not too much.
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:
I actually sleep on my back. Not that I don't believe you, but why should I try something firmer when I find this mattress to be very comfortable and never feel back pain? I assume that this causes a risk of future back pain - would you have any information about that specifically?
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.