A vinyl or material that is UV and cold-cracking resistant is the choice. Marine upholstery differs in that one has to consider dampness, sunlight and hard usage.
Stainless-steel hardware such as staples, screws must be used for a quality job that will last. Any wood used must be of marine quality.
Usually a high-resiliency, high-density plastic foam with a thin film of plastic over it is used to keep out water that might get by the seams. Closed-cell foam is used on smaller cushions which can double as flotation devices. Dacron thread must be used in any sewing work. Zippers should be of nylon.