After a heavy rain, followed by a few hot days, I went to my boat to remove the cockpit cover. There was some standing water drops on a few of the horizontal upholstered cushions, and after wiping it away, stains were left (See attached picture). First time this has ever happened to me; almost looks like bleach stains, but definitely just from rain water that leaked through the cover. Any idea how to get rid of this? I tried 3M boat upholstery cleaner, but that didn't work.