TLDR: Doug (may have) f-'ed up and made the "zinc" wrong on the boat, and they appear not to be doing anything. 4-5 years in the water, they should have seen some signs of corrosion. So assuming the zincs aren't corroding, it means something else is.
So - for those that don't know, metal boats in salt water require some extra care, as you can imagine - they like to corrode. Even "plastic" (fiberglass) boats need some help, as you'll have metal bits in the water (through hull valves, rudder posts, and ports that let water in/out of the boat for the engine, sinks, etc...).
So what you do is you add to the boat a "scapegoat", a "weakling" that the corrosive saltwater will "eat" first and spare your good stuff. (It's a chemistry/electrical thing) You do that by selecting a medal that is cheap, and something you can add to the boat and replace at intervals easily. You'll see them on prop shafts on sailboats a lot - that's a good place, as it's central to the boat's "ground" point. You'll even find them inside outboard engines. And right now, hopefully, there should be one in your hot water heater down the hall. It saves your tank from rusting. (they are replaceable too)
Commonly, they are called "zinks" - but you can make them out of other stuff. Sometimes they are made out of aluminum with a little zinc mixed in so that it dissolves evenly. But - you have to make them "doped" with a special element - or the whole thing doesn't work. The element is Indium. And it's one of the least talked about elements on the periodic table - but used in all sorts of things.
Dooooog being Dooooog, cast his own "zincs" from the aluminum scrap from the "Love me Tender" failed Jet boat, and although he added zinc to the mix (first with zinc power, and that didn't work, so he redid them with chunks of zinc mixed in) - he completely skipped the key ingredient to make it work. Indium. (source:
https://shura.shu.ac.uk/13482/2/Lambert - The Role of Indium in the Activation Accepted.pdf)
The problem with using aluminum, is aluminum likes to form a "skin" of aluminum-oxide on it (even in water) - The Indium breaks this skin, so that it actually works (more info in the academic paper above if you really want a rabbit hole.)
So as a boat owner, you really, really want to ask yourself the question: "If my "zincs" aren't corroding to protect the boat.... then what is???"
Oh, and hi Dooog if you are reading this.
(here is the casting video where he talks all about it if you want to watch it:
https://skiv.com/v/FE8kKF5-181125-Casting-and-Installing-Anodes-Learning-to-Weld-watermarked)