http://www.buckalgonquin.com/downloads/Cat_108.pdf
Page 88 has a bronze collar with set screw, but the shaft has to come out to install it. Split collar stainless is what you should have in salt, if you are going to use a collar.
However, the page 126 shows some Zinc anodes that should work on your shaft without working forward. Your shaft anode looks a bit different, and I wonder if you turned it around, if it would want to work toward the propeller. Personally, I would order a pair of the Buck Algonquin anodes and be done with it, I don't think they will move on the shaft. Pretty reliable company and product line.
Tom