Two choices, either do it yourself or send it away to be professionally upholstered. They would need existing seat back for plywood/template, etc. Do a Google search under Marine Upholstery.
If it's a standard bench seat back without ribbing then you can do it yourself. You'll need a sewing machine at the very least. It can work out expensive as you'll probably need replacement foam (available in different densities), the vinyl/acrylic canvas, piping for edges, etc. This company has online shop. http://www.cjmarine.co.uk/