Dad's '75 Sidewinder (455 Olds, Berk pump) had one. It was just a fin that hung down from the clean out cap pointing at the shaft. How well did it really work? Well I was a kid in the '70's so I wouldn't know the difference then but I can tell you that I went in the water a lot to pull weeds out of the intake so we could go ski. This was one of the downfalls to having a jet drive on Big Bear Lake (So Cal). What I also remember is Dad having to have it welded back on several times. Mom sucked a rope (more than once) and that would snap it off. In addition to that, if you sucked a fishing line (also happened more than once) that would snap it off. After a few of those type of incidents Dad finally gave up on it.
So in short, if you ran over a patch bigger than a Pole Dancer's you would clog the intake anyway. Anything other than weeds would snap it off. They stopped making them for a reason.