Best Fabric is SurLast® made by Glen Raven Mills who also makes Sunbrella. It was designed specifically for Mooring Cover application.
Most OE's use it but in a better construction "SurLast BW+"
MasterCraft, Malibu, Correct Craft, Centurion, Sanger, Supreme, MB Sport, Yamaha.
If your boat is fairly current you may be able to purchase a SurLast® Cover from your local dealer.
If not the fabric can be purchased from Sun Solutions in Indianapolis IN. (317) 209-8722