DO NOT USE A PRESSURE WASHER

The only time you can use a pressure washer is if you had a custom vinyl cover made. My old boat had one and it was 22mil thick. No way a pressure washer was going through that.

But most are sunbrella or another fabric like variant. Either way, they breathe. That's their design, to be tightly woven enought to allow water to not soak through when treated with water repellent, but still allow mimimum air movement.

By pressure washing them, you're opening up the fabric ever so slightly everytime you do it. Over time you will lose the ability to keep water out. Yes it won't let rain through, but moisture will begin to collect on the underside like dew, which creates a hot moist environment ideal for mildew and rot. Not to mention that when you pressure wash them you can damage the stitches are reinforcement points allowing a tear later down the road.

Grab a beer, spread it out, use a normal garden hose and sprayer, and scrub it with some normal soap and water. Should take you 20 minutes to clean it. Put it up off the ground on some lawnchairs or sawhorses to dry. Done. Every 3-4 years you should retreat the cover for water repellent. If it's constantly bombarded by weather (rain and snow) do it every 2 years. This will ensure you have good water beading and runoff.

DO NOT PRESSURE WASH YOUR COVERS.