Re: Upholstery foam..
Both of the boats that I've owned came with 4" thick bottom cushions (what are used as berths) and 3" thick back cushions.
Around 35lb foam and the highest density that you can afford is good for the bottom cushions. We put a 2" memory foam topper on top to make it very comfortable. We also use this to bridge the gap between the cushions. We roll them up for storage.
Back cushions are softer (maybe 20lb? It's been a while).
I bought foam in standard mattress sizes and cut it up myself using an electric carving knife. A cheap one works fine (just give it a break once in a while). In general one twin mattress can be cut into the two V-berth cushions. My two quarterberth cushions were cut out of a queen sized mattress.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior