I dwelt with this last fall. The boat I bought had sat in the water behind the PO's house. He was unable to maintain the boat and I believe let the zincs go for some time. We noted some of the problems during survey but after purchase, I had the boat hauled for some work and the extent became much clearer. The "excitement" of the discovery got the best of me and I believe I overreacted to the yards recommendations. Ended up replacing both struts, both main thru hulls and seacocks, both rudders (one was bent anyway). Props were fine. I also had a marine electrician investigate the cause and he determined that my bonding system had failed. Many of the wires had corroded. I replaced all using marine grade wire and terminals. Then after splashing, all items were retested by electrician at my slip.
After the work was done, I did some additional investigation. One of the struts was truly bad. Drilling in several locations it showed pink about a 1/8' deep and sounded a dull thud when struck with metal hammer. The other strut was truly just surface pink and sounded with a ring. It was going to have to be removed and realigned for the shaft anyway so I had decided to order a new one and replace it. I am keeping that as a spare now. The bent rudder was condemned and not able to be straightened. The other rudder was just pink on the surface and determined to be fully serviceable. The seacocks were both leaking and were not economically rebuildable per the yard. I ended up replacing a strut and rudder that I did not need to.
Lessons learned:
I don't think the my yard acted maliciously but when the surprises happen, I can't let the heat of the moment and the percieved pressure of the yard force me to do anything until I have time to investigate and satisfy myself.
Electrolysis did not effect all components equally. Each item needs to be investigated and a cause determined.