Would different material be used for a berth?
You need some beathability in a berths but the same could be said of all cushions. They used to use waterproof bottom material in berths but it just traps the moisture. If the material is waterproof on the surface you'll wake up wet. If the material breaths well it won't condense on the bottom or top. You could expect some moisture to accumulate as you sleep so how it can get out is the key. It's why traditional inner spring mattresses rust quickly on a boat.
The best I've found is 6 inches of quality foam under 2 inches of memory foam with a breathable cover and bottom. It's a slightly firm sleeping mattress but so far no condensation
issues. This sits over a varnished wood surface. You'll get condensation
on that if there is a problem with venting and we don't.
You use to be able to buy very nice closed cell foam. You can't get it any more as the one plant that made it burned to the ground. The stuff you get now is hard as a rock. The old good closed cell was a full 2 inches thick and that slept well if you had enough material over it. More like the high quality wrestling mats used in college and high school
. A little firmer than I would choose today but some years ago I slept well all night. The key is the quality of the foam first as far as sleeping. Cheap foam flattens quickly when you sleep on it in a few years - honest.
If you use expensive material to cover the foam you can still expect to pay more for the foam than the material. Foam prices are through the roof these days and follow the oil
market prices close. There really are no cheap deal unless you luck into a close out or some odd situation. You can layer foam to get the feel you want and it is no more expensive to do that. A little memory foam over good firm foam will sit and sleep exceptionally well for a long long time. It can feel as luxurious as anything else.
The custom inner spring boat mattresses are nice too. They use a stainless spring so the rust is not the issue expected using normal mattress springs. That will sleep more as you do on land. They are custom made so they cost a bit more too.
It comes down to how well you sleep. At 30 I could sleep on a pile of bowling balls. Now it's not so easy. The older you get the harder it is to sleep well. Sleeping well matters a whole lot all the time.