I recently made cushions and covers for my Bayfield 25. I'll share the "experience":
1) The foam was about $600cdn for v-berth and two seaberths. Got it from "Foamite". Top-quality stuff, great service. Four inch thick for seaberths, five inch for v-berth - Medium-High density. I found the High density too stiff. Sleeping on this stuff is like sleeping on a marshmallow!
2) Make a pattern out of card-paper and shape the foam with an electic carving knife. (Like the one you use for the turkey)
3) Zippers should be plastic.
4) Seaberth covers made out of vinyle. I did this because on a small boat, everything gets wet, and the waterproof covers on seaberths work great. Only problem is they don't breathe very well, and any uncovered skin will "stick" in the summer
... That's a trade-off I guess. Oh, and whatever you do, don't buy that "marine grade stuff"!!! I paid 1/4 for basically the same stuff. Fabricland!
5) I was thinking of using memory foam (Tempur-pedic), but the lady at foamite said it would be too "clammy" in a moist environment because it is so dense or something like that... Anyone have first-hand experience with memory foam on a boat?
6) As for the covers, I somehow convinced mum to sew 'em for me. We used the same card paper patterns that I had made for the foam cutting. We traced each pattern onto the fabric, one for top of cushion, one for bottom, then cut a 4" strip for the sides. I know nothing about sewing, but she was able to put these together in a few hours after I had cut everything out! (leave a half inch or so extra around all edges!)