I have one of those custom mattresses like Sck refers to and they are indeed wonderful. I would invest in one if you are living full time aboard. For weekends/vacations, you can get away with a lot less and I'd suggest going to a foam shop OR someone who restores autos to build new foam bedding for you AND/OR use the underlying FROLI spring system to provide an added measure of comfort and moisture proofing. If you have servicable cushions...you might try the Froli before investing in new ones.
Froli Sleep Systems by Nickle Atlantic, LLC