To protect and restore my vinyl upholstery, I use Starbrite Vinyl Shampoo as a first step because it removes dirt, stains and embedded grime. The best protection for my upholstery is good maintenance. Always clean upholstery thoroughly after a day on the water, using fresh water and a towel for drying it. Vinyl repair kits can handle most small tears and rips. A small tear can be glued together using only the vinyl repair kit.
Larger tears are likely to require professional repair services.
If the upholstery can be removed from the boat, take it to a marine or auto upholstery specialist who can sew the upholstery properly. If you're feeling a bit more adventurous, then a do-it yourself reupholstery project is for you. A sewing machine will be needed, but the basic skills are easily learned, and there are excellent books on auto upholstery that provide all the tips you need.