You might try repurposing some cushions: try an outdoor / patio section at a home repair store like Lowes. Look for the indoor/outdoor cushions for swings. They are usually 48-60" long, and they are made for outdoor use. If you take them in after every use, you could use indoor cushions for a bench or a window seat. All my cockpit cushions are about 70", so my wife bought several of these cushions and sewed them together. They work great, and they cost very little. Online options exist too. In fact, I think my wife ordered ours online from a place selling outdoor cushions.
Oh, and any cushions from an old boat are likely to be as old, and stained-er and stinky-er than the old boat!
Thanks,
Andrew