I would respectfully disagree with Ian's suggestion that you not worry about it. The zinc on or close to the heat exchanger serves to protect it, not the prop and shaft. Similarly, the shaft zinc serves to only protect the shaft and prop. The efficacy of an anode is very sensitive to both electrical distance and path resistance, both of which combine to make a shaft zinc useless in protecting the heat exchanger.
If Yanmar designed the SVE8 to have a zinc anode, it's worth the slight trouble and the $6 in cost to find and replace it regularly.