The last set of seat cushions/sponges I replaced are for the flybridge seating. The original cushions were foam with a zippered vinyl cover. The zippers were not watertight and had rotted away from the covering. When it rained (this is Seattle), water soaked the foam cushions. They were essentially not useable. Plus they stained easily. So, like the new bow seat cushions, these are made of closed cell foam that is waterproof on its own, and are coated with vinyl for another waterproof layer and finishing touch. Here are some before and after pics. I think they turned out great and, unlike the old ones, are useable!
Fiddler 