Simon is pretty much on the money if only because a box spring mattress breathes which is something no foam mattress will do.
OTOH if you can't spring at the box mattress, on our previous boat I had berth cushions made up of a layer of very firm high resilient foam with an overlay of very soft HR foam and two layers of Dacron fibre sheeting. I then had the whole thing encased in mattress ticking. 20 years later that mattress is still giving sterling service for the current owner. For an odd shape (e,g v-berth) it slightly more practical than BSM if only beacuse you can still have three separate cushions incudling a removeable filler piece. I used a similar concept for the settee cushions. The great advantage of the Dacron btw is that is gives a measure of breathability.
“Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life.” Terry Pratchett
Last edited by tdw; 09-11-2007 at 07:00 PM.