AnnapolisStar, you must have an older edition of Nigel Calder's book, I've got the third edition and it is correct; his procedure for testing the integrity of bonding circuits is on p. 218, and there is a comprehensive test on p. 234. Page 216 is about sizing zincs for your boat and should be read. Actually, the entire chapter is well worth reading. Heck, just read the whole book.
You are correct that a zinc can be used as a reference cell! But cathodic protection systems use zinc (or aluminum or magnesium) anodes to purposely generate a current - the forced current protects your boat's metals by causing the anode to deteriorate instead. A reference cell allows you to measure the voltage between your protective anodes and underwater metals because there is current flowing through the water between them.
Zinc makes a very convenient reference cell since the table for an Ag/AgCl reference cell can be used, just add 1 to get the zinc reference cell value. The third edition of the Boatowner's Mechanical and Electrical Manual has a table of the Galvanic Series of Metals in Seawater on p. 199. From the table, the "natural" voltage of an electrically isolated bronze thruhull as measured with a Ag/AgCl reference cell should be around -0.3 V; with a Zn reference cell: -0.3 + 1 = 0.7 V.
Using my fiberglass boat as a real example, with an Ag/AgCl reference cell my hull potential (and every bonded underwater metal) in seawater is -0.96 V. Using the Zn reference cell in my picture below everything measures 0.09 V.
Remember, that table has values for SEAwater, if you are in brackish of fresh water the values will be different - Google for the appropriate table ... and notice the relative reference cell - there are several types!
I made my reference cell with an engine pencil zinc. You need a good connection between your wire and the zinc so drill as small a hole as possible to fit the (stripped) wire into the hole. For this one I drilled almost 0.75" deep, and slathered the wire with an anti-corrosive conductive paste before cramming it in the hole.
The wire is held to the zinc with "Rescue Tape", which also does a perfect job of keeping the connection dry. You can put a meter jack on the other end, or crimp on an alligator clip such as those in the picture, and clip that onto your meter's probe. Be sure you have very good connections to keep resistance to a minimum as you are measuring very small voltages.
In salt water you could measure the whole boat by positioning an Ag/AgCl reference cell amidships about two feet below the surface, but fresh water is not very conductive so it needs to be within a foot of the metal being measured in fresh water. In salt water, a zinc reference cell needs to be within a foot or so of the metal being measured, I do not remember if it will work in fresh water.