I am well over 250 lbs and have back problems. Soft foam is a death sentence for me. My chairs at home were all filled with normal foam. Normal foam for settees is way too soft for me. It's important that you customize the foam for your own personal needs. Foam comes in various densities. I chose to use a firm V44 foam for our
boat. That should serve people over 200 lbs well. In my private armchair and dining room seat I got even firmer material (V54). I am a happy man with no more back issues from sitting on soft chairs with no support.
I plan on making
cockpit cushions too, but in that case, you need to use closed cell foam or waterproof covers. The thickness is also important. I'm very tall. Your position sitting is key to longterm comfort. You should not sit so you are either bending your leg up or down too much. Your weight should be evenly distributed along your legs and buttocks. I increased the thickness of my armchair cushion to raise me up by two or three inches. The chair is a bit high for average people, but perfect for me. Same should go for the
cockpit cushions. Instead of one long cushion, you may want to make several separate cushions of differing thicknesses to serve your different crew members.
If you are looking for a good supply house, check out Foamorder.com
Foam, Neoprene, Cushions, Latex Mattress, Outdoor Furniture Cushions, Sunbrella Fabrics. This is where I bought my foam for my two chairs. My upholsterer got foam from his local supplier. But we asked for firmer stuff. If you are sitting for a long time there is nothing worse than foam that is too soft.
Getting something too hard would be equally bad, just try sitting in the
fiberglass cockpit for a long sail sailing and you know what I mean.