While the purpose of a boat cover is to keep your boat clean, if you don't keep your boat cover itself clean you could end up with quite an unsightly mess parked in your driveway or in the local marina. The good thing is that since most boat covers are designed to be water resistant, you shouldn't have much of a problem with mold and mildew. However, these covers are exposed to the elements, which subjects them to all manner of dirt, which can hold tight.
Spray down your boat cover with lukewarm water using the nozzle on your garden hose. While the cover is on top of your boat, start at the top and move left to right down the right side. Then repeat on the left. This should help get rid of a lot of the salt if your boat is exposed to seawater.
Take the cover off your boat and lay it open on the ground with the side that's normally exposed to the air face up. Brush the cover with your push broom, starting at the left edge and moving to the right edge. This will remove any remaining large debris. You may need someone to stand on one edge of the cover so it doesn't roll up when you try to brush it.
Fill your bucket to its halfway point with water and pour in a quarter cup of a gentle soap. Do not use a detergent, since it can damage your cover's waterproofing. Stir the solution and dip your sponge in it. Use the sponge to dab any areas of your cover that are stained. Let it set for 10 minutes.
Scrub your cover with your scrub brush using a circular motion until all of the stains are removed. If it takes you longer than a half hour, switch from your gentle soap to a color-safe bleach, using the same measurements as before.
Rinse your boat cover with the hose using the method in Step 1. Then let it air dry. If you have any spots on the inside of your cover, flip it over and repeat Steps 1 through 4 to get them off. Do a final rinse. Then let the cover air dry by laying it flat or hanging it over a clothesline.