The point is that YOU may not have the problem, but you WILL be effected by anyone else (or Marina) who has a problem.
All you can do is make sure your anodes are adequate, and bonded in (check that there is a circuit) to the bits you want to protect. If you lost 1/3rd of the anode in one year - don't assume that the other 2 thirds will be sufficient for the next year. Put a new one on and when the guy next door is leaking electricity like battersea powersation, you will have a bit of time before your prop falls to bits!
Dezincification selectively removes zinc from the alloy, leaving behind a porous, copper-rich structure that has little mechanical strength. An in-service valve suffering from dezincification has a white powdery substance or mineral stains on its exterior surface. The valve may exhibit water weeping from the valve body or stem/bonnet seal.
Why Dezincification Occurs Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. Zinc is a highly reactive metal, as seen in its galvanic series ranking. This reactivity stems from the fact that zinc has a very weak atomic bond relative to other metals. Simply, zinc atoms are easily given up to solutions with certain aggressive characteristics. During dezincification, the more active zinc is selectively removed from the brass, leaving behind a weak deposit of the porous, more noble copper-rich metal.
Two types of corrosive attack characterize dezincification: plug and uniform (or layer).
-Plug-type dezincification is localized within surrounding surfaces mostly unaffected by corrosion. This type of dezincification penetrates deeply into the sidewalls of valves and fittings. Common failures associated with plug-type attack include penetration through the sidewalls that causes water seepage or loss of mechanical strength in threaded sections to the point of fracture.
-Uniform-layer dezincification leaches zinc from a broad area of the surface. This type of dezincification uniformly reduces the wall thickness of the valve or fitting. A complex set of conditions must be present for dezincification to occur, and the occurrence is often related to region of the country.
The service conditions generally present where dezincification occurs include:
-Water with high levels of oxygen and carbon dioxide (uniform attack).
-Stagnant or slow moving waters (uniform attack).
-Slightly acidic water, low in salt content and at room temperature (uniform attack).
-Soft, low pH and low mineral water combined with oxygen, which forms zinc oxide (uniform attack).
-Waters with high chloride ion content (uniform attack).
-Neutral or alkaline waters, high in salt content and at or above room temperature (plug-type attack).
Common signs that dezincification is occurring include:
-Presence of a loosely adhering white deposit of zinc oxide on the exterior of the valve.
-Presence of mineral stains on the outer surface of the valve.
-Water weeping from the valve body or stem/bonnet seal.
I would agree with Chris. Replace your anode each year and compare the amount of corrosion that has taken place. Keep an eye on your propellor and metal skin fittings. Hopefully, an alert owner would be able to spot any problem before it became too serious.