When you're back in Brunswick let me know, we're on dock 14 in BLM.
First, I'm not an expert in bonding systems. But here is what I understand.
If you connect different metals together with a conductor, one will corrode (the anode and less noble metal), and the other will not (cathode and more noble metal).
Seawater will conduct electricity.
When seawater is in contact with different metals (say a stainless steel pump impeller and a bronze seacock) one of these is less noble and will corrode (the anode). Zinc is more anodic than the other metals on your boat. By connecting all the metals that are in contact with seawater, and then connecting that wire to the zinc anode on the rudder, the zinc will corrode and protect the other metals in contact with seawater.
This is a form of "cathodic protection", check wikipedia for a more detailed explanation of cathodic protection. This is an electro-chemical reaction.
Wanderer, SM #477