Simple answer is this.
Zincs stop electrolysis, which occurs in the water. Once you remove a piece of metal from the electrolyte, (usually salt water in a boats case) you stop the process.
The corrosion issues you described above are caused by oxidation, so you would have to remove the oxygen from contact with the metal.
This is done on the atomic level by using the galvanizing process or stainless steel.
Hope this answers your question.
Jon Caisson Bozeman
S/V Little Miss Magic, Pirates Cove, AL