I know what you're talking about with the firm mattress(foam) Four winns uses. We ordered this topper from overstock, and the difference is unbelievable! It makes for a perfect combination with the foam being firm and the topper being so plush. We liked it so much that we tried it on our pillow top at home after the boat was in storage, but it actually made it too soft and we both ended up with aching backs.
http://www.overstock.com/Home-Garden/Serta-Deluxe-2-inch-Memory-Foam-Mattress-Topper/1080221/product.htmlAlso speaking from first hand experience it is so nice living close to your marina. We live about 10-15 min from ours and I'll go straight from work to meet my wife for dinner and a sunset cruise at least 2-3 times during the week! Also being able to go to your boat after a bad day at work and crack a cold beer is priceless!
We went with a 4" memory foam topper and loved it. Even bought a second one for the house. I didn't go with the Serta...but with the cheaper brand. No complaints with it on my pillowtop at the house.