I think the problems with sprung mattresses on boats are likely to be (a) boat bunks are often odd shapes and sizes and (b) would the springs not rust? ***
IKEA is great for these kinds of things - lots of variety of foam formulations and thicknesses, and sizes, and cheap so no second thoughts about getting the big stanley knife out for some surgery. And good quality too, in our experience. The covers are fairly easy to amend too. Finally, as Brendan says, the slats - also a really good, yet cheap, product - have been highly recommended over the years here.
sweet dreamzzzzzz

***Edit
ok, I've looked at the link marine bedding website and obviously this is a pretty serious - high price? - concern. Luxury. Bespoke for shape. Springs probably won't rust. Ignore first para then.