Vinyl boat seats wear out when they are constantly exposed to the sun and other outside elements. If the elements don't get to them, age will eventually damage the most well-cared-for boat seats. Replacing them with new ones can be pricey. It's cheaper to reupholster them yourself and it isn't that difficult. If you can cut, measure, use a few basic tools and sew straight lines, you can make your own.
Remove the boat seat from its frame. You'll probably have to use a screwdriver to unscrew the seat. Place the screws in a secure location so you don't lose them.
Remove the old vinyl from the seat. If the bottom isn't attached with staples you can pry loose, you may have to remove the fabric with scissors. Be careful not to damage the under layer.
Measure the seat so you'll know how much new fabric to buy. Measure the width, length and thickness of the seat. For example, if your seat is 24 inches by 36 inches and it's 4 inches thick, you'll need two panels of fabric that are 28 inches by 40 inches. An extra one-inch seam allowance is added to all sides when sewing the panels together.
Cut your vinyl to the measurements. Place your two panel's right sides together and pin them together on three sides, leaving open the side that will be the bottom.
Sew the three sides of the panels together. Turn the fabric right-side out. Place the panel over the seat, making sure it fits snugly.
Fold the bottom edges over and staple them down. Place your finished seat back on the frame and screw it back into place. Repeat these steps with the other seat covers and reinstall them. You now have reupholstered boat seats.