When buying a boat cover online, purchase one based on its purpose. Some covers are designed to attach to trailers, while others are rated with secure ties for higher speeds. If you plan on taking the covered boat directly into water, be sure that the ropes go underneath the boat's hulls while it is on the trailer, to prevent moisture damage.
Do not choose a cover based on style alone, as the durability of the cover material is more important. Any detailed prints or graphics on the cover may get washed away after extended use and exposure to rain and the elements. Most covers are also designed to only wrap around the hull's top, which means that you may need to invest in specific options for driving platforms or motor coverage.
Choose a cover based on your local area, as not all covers are designed to be entirely waterproof. Most are designed to simply resist water and create a gliding effect for rain and frost, but if you live in an area that experiences heavy downpours, purchase a cover that is exclusively waterproof.
There are numerous material options available, and these options significantly affect the way the cover functions. Older covers are typically made from cotton canvas or "duck" materials, while newer covers are made with synthetic acrylic or plastic, to increase their ability to resist water and moisture damage.