The v-berth bed was made up of multiple cushions which we found uncomfortable. It seems like you are always laying in a crack and there is no way to make the sheets nice and tight. We decided to order a new mattress and debated between memory foam or an inner spring mattress, but ultimately decided on the foam. We thought it sounded more comfortable, was cheaper, and easier to dry out if it got wet. We read in a few of the sailboat forums about the Spa Sensations memory foam mattress from Wal-Mart. As much as I can't stand Wal-Mart, the mattress was super cheap and had 3700 positive reviews. We got the 8" Spa Sensatins Memory Foam Mattress for $260 (it's dropped to $199 now of course!). It comes with a nice zip up cover on it and is so comfortable!
Matt measured the dimensions of the v-berth and then drew it all out on the mattress. Then he cut it with an electric bread knife which was really easy and doesn't tear it.
You can see the mattress is 3 layers. It's memory foam on top and then higher densities on bottom.
And it fit perfectly! I cut and resewed the zip up cover (see here for that post) so that it fit the new shape. Check it out! It's huge! It's as wide as a king size at the head and goes down to about 20 inches at the foot. Loads of room for 2 people!