I didn't have any seams; my cushions are narrow enough that I was able to cover them with a single piece of fabric each. The sides are just folded over extensions of the top. If I did need seams, I would fold the edges of the fabric over 1/2" or so to hide the cut edges and butt the folds together.
I used 1" closed cell foam which is adequate for my small boat. It is harder than regular foam and not comfortable for sleeping but is adequate for seating
my ample behind. For a fatter and softer cushion you could make a sandwich of regular and closed cell foam. The layers can also be glued together with contact cement. I would not use the foam designed to drain water right through in this application as both the Sunbrella and the layer of contact cement are relatively waterproof.
I used regular contact cement that comes in quart cans from the hardware store. It is usually used to apply laminate to countertops. You paint
it on both surfaces (the fabric and the foam) with a disposable chip brush (NOT a foam brush). The trick is to let both surfaces sit for about twenty minutes until they are dry to the touch, then put them together. They will bond instantly so you have to be sure they are properly in place.
To make my cushions, I first cut the foam to shape. I used a piece of cardboard on the boat to make a template to get the curves right. I laid the Sunbrella out on the floor good side down. (Sunbrella does not normally have a "good" or "bad" side but I got a bargain on some fireproof stuff on ebay that definitely is different on each side. The finished side is a perfect color match with regular Sunbrella.) I laid the cut foam on top of the fabric and traced its outline about 3" wider than the foam all around. I cut out the fabric on this line
. The extra 3" allowed the fabric to fold over the sides of the foam and onto the bottom a little.
Then it was just a matter of painting the contact cement on every square inch of the "in" side of the fabric, on the entire top side of the foam, and on the sides of the foam as well as a few inches along the bottom perimeter. Lay the fabric out on a flat floor and when the contact cement has dried to the touch put the foam on it. Make sure it is aligned correctly as it will bond instantly and is hard to remove or adjust. There should be 3" of fabric (more if your cushion is thick) sticking out all around the foam cushion. Fold this fabric up to form the sides and ends, and then fold what's left over the bottom. Remember, the cushion is upside down. You might have to make a few slits in the fabric on the bottom side or double fold on the curves to make it lay flat. The finished side of the cushion will be as perfectly flat as the floor you are working on.
I'm no expert; this is just what I did and it worked for me.