I tried a couple of different kinds including the kind that suction cup onto the side (they looked cool, long slender, like something that would protect my fence tin). Well, they didn't work very well, so I like many people have the standard round cylinder type with a long adjustable rope (about 4' so I can adjust the height of the fender to suit the dock). But I have found there are so many docks that seem to be the perfect height for scratching your stuff, that it I just hate to tie up to anything unless it a pristine dock with perfect padding on the corners. Since that is not always practical, my kids get the credit for coming up with a great idea!
We now often take a spare life jacket or two and actually attach it to the problem post or object that looks like it is going to "rub us the wrong way"
It seems to help. One other thing, I always keep two on either side, but when facing a troubling looking doc we also bring all 4 to one side to help protect.
In my opinion this is the only thing I don't like about pontoons. Those sides are so soft and easy to scratch or dent that if you screw up even just a little, your machine get's scratched!

I have about 5 "boo boos" on my brand new boat and it makes me frustrated every time I look at them.
I guess that makes me either unqualified to answer this question (because obviously I have not been perfect at this) or it makes me an expert because of the fact I don't have 75 "boo boos" instead of 5.
Good luck, and like I have said, count to 10 when you screw up and drive the corner of your fence into the edge of the dock because you "thought you were da man!"
p.s. Here is a picture of my brand new main L couch seat that my kids "stood on top of" when I wasn't around and tore the seam! (I had to count to (1000 by 10's before I got over this one). Credit goes to Gary at Voyager Marine for sending me a brand new upholstry cover for this seat (I had to remove the old one and replace it, but he sent me the material for free). It looks good as new now. Wish the same was true for my scratches and dents!
So