Lower back fatigue can set in after a few hours of sitting on a fiberglass bench, not to mention buttock fatigue. I don't have bench cushions on my boat, but for four bucks at a garage sale, these Kelty seats give good back support and the Bleacher Creature seat pads make for a much cushier ride. I don't know what they go for new, but it can't be much. A good inexpensive option that's easy to remove and store.
(http://i901.photobucket.com/albums/ac215/Sterling100/image_zps928cb118.jpg)
They're around $15-20.
Sterling,
It looks like you sail a 16, as do I. How does the depth of the Kelty seats match up with the depth of the 16 cockpit benches?