All boats are electrochemical batteries. The different metal parts sticking into the seawater interact with each other, pick out a loser, and pull atoms off that part into the ocean. Eventually, the losing part disappears altogether.
Engineers, techs, and wandering drunks have some fairly serious disagreements about the details of how to address this metallic warmongering, but generally we connect all the metal parts together inside the boat, where conditions are more favorable for metal parts, and we put a piece of zinc or magnesium on the wet side of the boat that takes one for the team. The sacrificial anode purposefully becomes the weak link, protecting the other metal parts by giving itself to the sea. We replace the sacrificial anode periodically to make sure the important parts are protected.
The only sacrificial anode on Bright Water is a zinc collar mounted on the shaft just forward of the prop. It lasts about six months between replacement, which is fine. However, the electrical path between the anode and the sea and the rest of the boat runs through the bearings of the transmission. Typically you don’t want to run current, even tiny current, through moving parts. The current can cause micro-pits which become macro-pits which become major pits. Then the money flows like water.
We installed a shaft brush on Bright Water. A conductive graphite pad is pushed against the stainless steel shaft, and a wire connected to the pad carries current around the transmission bearings directly to the engine block ground post. This is pretty normal stuff, but an amazing number of boats don’t bother with it.
(Edit) Here’s a pretty good summation about the bigger picture, although the ground plates that he talks about have been discredited because the additional surface area of the sintered plate contacts exactly the same amount of seawater as a solid plate of the same size: http://www.westmarine.com/WestAdvisor/Marine-Grounding-Systems
We avoid this added complexity by never, ever, ever plugging Bright Water into shore power, especially in Mexico. I don’t think we even have a power cord on board. Never swim in a Mexican marina. If the docks aren’t screwed up, some of the boats in the marina will be. Based on what I’ve learned, I wouldn’t swim in any other marina, either.
If we get a SSB or HAM radio, we’ll have to do something about RF grounding, but I’m leaning towards a counterpoise instead. Lots to learn.