I bought 6 fender covers two years ago. Can't remember the brand but they were made of a thin blue knit synthetic. Pulled them onto the fenders, tied the boat alongside the slip, made everything tiddly and left.
The boat was slipped at a fixed dock in a Great Lakes marina, very sheltered. No tides and no turbulence.
Came back three days later. The covers on the three fenders between the boat and the slip were all bunched up at the top of the fender. Pulled them back down and looked at the covers on the other side. They were all in place covering the fenders. Not surprising as nothing was touching the boat on that side.
Two days later was back at the boat. One fender cover missing. Two others had ridden up the fenders again. They were starting to get quite dirty from the gunk that coated the side of the slip. It had deposited itself on the fender cover and the air had dried it out. Crusty is prbably the best term for it.
The covers on the other side were fine. Just where they had been put and no Lake sludge.
Took the dirty ones off the fenders, threw them into the wash at home and bought a new one to roekace the missing one.
They rode up and got dirty again. They got very, very crusty this time .
I took them off the fenders and threw them away. Won't buy them again.