From Sunbrella’s Web Site: (Link)

Powerboats and sailboats need Sunbrella®, the #1 marine fabric in the world.
Sunbrella® fabrics are the strongest in the business, which is why we’re the most trusted fabric for biminis, dodgers, winch covers and cushions.

Made from solution-dyed acrylic yarns, the Sunbrella® name is the best performance fabric in the business. When your reputation depends on quality, Sunbrella® is the obvious choice. Sunbrella® fabrics look great even in the most destructive environments: salty wind and water. They also resist tearing, shrinking, rot and mildew. Plus they are woven, which makes Sunbrella® breathable, so it doesn’t trap heat and moisture like vinyl. When your customers request fabric with extra water repellency, recommend Sunbrella Plus®, the same great fabric with enhanced protection from the rain.