Yes I agree whole hartedly fstbttms.
You can do a simple continuity test to the shaft and see if there is a good connection. If not, then a shaft brush can be used to connect the shaft with the rest of the engine
. However, although "fish" anodes do work and are certainly better than nothing. A major deficit can be had with the use of these. Firstly, you need to have the anode itsself within a few feet of the metal you are protecting. Sometimes not easily done if hanging over the side. Secondly, there must be very little resisitance in the cable used to connect to the metal. So the longer the run, the heavier the cable needs to be, or the resistance in the cable will drop too much voltage, and the metal will no longer be protected correctly. Often in some boats, like fibreglass, that run can be all the way from over the side to down into the engine
room, which can be a long way and a pain at being in the road of things like hatches.
Now in saying all that, I use a "fish" as a monitoring anode. It works well. so I am not saying they are entirely bad. Just some thoughts to consider.