Hmmmm. Cloth would breathe, and have better drying potential. It would require the cover to be left off for a while though, but they really should be opened up to dry anyway.
Maybe a combination of that's how it's always been done, already having the accounts with suppliers, and protecting wood framework. A lot of seats, cushions, and side pads still have wood. The vinyl will repel the water, where the cloth would just soak it up to dry later. The foam padding is also going to soak it and be hard to fully dry.
To use cloth, say on a back rest cushion mounted to the boat, a plastic back with lots of vent holes and the mounting tabs that just slot into holes in the backing ( so the cushions just lift off), might work.
Another issue though would be tearing. Boat cushions are usually wrapped around the edges of the backing and stapled, vs your patio furniture where the fabric likely wraps around completely and is stitched together. It could still work for fully wrapped stuff like seats, and might work for cushions if the edges had a reinforced hem sewn on like my couch.
