"Well, I'll be damned if it isn't a fender! Those things are $30 apiece

You would think people would tie them on better and pull them in the boat when
they are underway. I have never bougfht one but I have about a dozen I found
stuck up in the mangroves. I use them for pool floats and whatever else I can
think of. No more room on my boat.
BTW the same goes for type IV cushions.