I'm not sure why you would use the Silver Chloride Cell. Presumably you could go around the hull at varying distances and make a contour map of voltage measurements. Unfortunately there is no way to tell if the voltages are caused by your boat or other boats and fixtures in the vicinity.
From that you can deduce that you have a galvanic problem, DUH you knew that already. It might show you areas of very high concentrations that you "might" be able to attribute to an adjacent boat or dock
but you still have a galvanic problem despite knowing the origin. There is hardly any way a zinc will not work and diving
on it to inspect will tell a lot more than a table of voltages.
Better to spend the $125 on a Galvanic Isolator
and some zincs and work on protection rather than diagnosis.