Re: Upholstery foam..
When we did ours the woman doing the cushions gave us a choice of soft-medium-firm-extra firm. She recommended firm for the berth cushions and firm for the salon cushions. She was right on the money.
As for the memory foam toppers some love them, I find them hotter to sleep on and less comfortable YMMV.
95 Catalina 30 Island Time
“If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most" - E.B. White