Futon would not work.
For all the upholstery, curtains, and mattresses in the boat you want NO natural fibers what so ever. They quickly become pretty stinky and don't wear well at all. Wool may be the worst but cotton is right up there too. Towels and sheets
in cotton are fine since you can wash them regularly.
The shop that did our boat has a story about a customer that was insistent they wanted the fabric
all to be "blue jean cotton", After three years they ripped it all out. Not because of a fashion whim but it stunk so bad.
We really like a high density foam under a layer of memory foam. High density foam isn't cheap
but it will last a long time and is used in commercial
grade upholstry. Cheap foam breaks down very fast. Don't use laytex it's a natural product too. We cut the thickness down a little bit for the rest of the cushions
If you get some help you can make a custom combination of layers to make it really comfy and maybe save a little money