Try putting a egg crate foam pad over the mattress in your boat. This works wonders for me and I like it a lot. Get a descent quality one, take your cushions out of the V-berth and set them on top. Then trip the egg crate mattress to fit the cushions and put these in the v-bert. There you go, all set. Cover with sheets for sleeping and you are good to go. It adds a lot of comfort to the bed.
The only other thing that I have heard of is much more expensive. You can have a canvas shop make you a new mattress out of several layers of memory foam. This is also expensive. Try the Egg Crate mattress idea first because it is a $30 fix.