Like other have said, you need to specify the density and that depends on if
you will be using it as just berth or if it will double as a seat. Seat
cushions foam will be denser than bunk foam. If you want a mattress that
will help hold you in place while the boat is under way, get something
softer than what you would want if you spend most of you nights at a marina
or a anchor.

Seat and cockpit cushions are usually only a couple inches thick and should
be firm enough so your tail /nip bone don't penetrate too deeply. (you know
what I mean.) If you have high lee-rails on the edge of your settee, then
their thickness should come up to or slightly above the rail so you won't
cut off circulation in the back of your thighs.

Don't ignore the option to have a softer foam with a firmer foam layer on
top. (or is it the other way around) This should be glued to keep it from
slipping around in the mattress cover.

The guy in the foam store won't be much help in either instance (my
experience). He generally only knows what works on a home bed or seat. And
if he 'thinks' he knows what to use on a boat, he is thinking Harbor/Marina
Queens. Not sea going vessels.

Be prepared to pay more than you expect for good foam. I paid about $200 for
enough foam for my fwd Vee berth. (you pay for the wasted material for odd
shapes.) The store is best suited to cut you foam if you bring in a
cardboard pattern. However, they will generally only cut around your pattern
and will not make an attempt to cut that angle you may want where the
mattress fits to the flare of the hull. Make some allowance for that and cut
that yourself with a serrated bread knife.

I have seen a special mat material that is designed to be under your
mattress for ventilation. It is like a plastic straw mat and I saw it at the
marine supply store. Seemed like a good idea but you would have to reduce
the thickness of your foam by that amount.

The fabric for covering your mattress's is another topic but I prefer to
totally cover all round with fabric so they can breath. If they might get
wet, then put on a plastic bed sheet on that bunk, while under way. Quarter
Berths are always getting wet from the companionway hatch.

There is a supper expensive foam that it totally open cell and will allow
the water to drain all the way through without being absorbed by the foam
material.. For this stuff, be prepared for "Sticker Shock" About double
regular good foam..


--
My experience and opinion, FWIW
--
Steve
s/v Good Intentions