Usually the original seats for your boat are available for around 5 years after the boat models release date, but will vary from manufacturer to manufacturer. These seats, if available, can be ordered from a certified dealership, and are typically on the expensive side.
If you're boat is older than 5 years (or however long the manufacturer makes them available for) or you don't want to fork out an arm & leg for the OEM replacments, the best option to upgrade or restore is either to purchase marine-grade vinyl and have your seats professionally re-upholstered, or purchase after-market seats. Typically, re-upholstering seats is cheaper, but can get expensive depending on the condition of the seats' foam. If aspects of the seat other than the vinyl are questionable, we suggest replacing the entire seat.