Vinyl hot tub covers typically are treated with UV, cold crack, and mildew inhibitors. These treatments help preserve the life of hot tub covers at the same time the cover preserves the life of your spa.
Deterring mold is a very important part in protecting the life of hot tub covers because mold makes them susceptible to wear and horrible smells. There is nothing worse than a smelly hot tub cover.
You can get hot tub covers custom made to your own spa, or just choose from the many options there are available to you. If you live in an area of extreme weather temperatures, you may want to look into purchasing a thicker, more durable cover. But for most people, a typical vinyl hot tub cover will suffice. They are strong enough, and unless you are expecting heavy snow or rainfall, this type of cover will work for you.
Aside from the fact that hot tub covers protect the inner-workings of your hot tub, they also protect it from plant life, mold, insects, and prevent animals and children from falling in. A lot of spa covers have locking mechanisms that add to the protection they provide.