Unfortunately, the plastic bowls on strainers seem to be a problem area as we've discovered several times (to our dismay).
PS: We just had a look at your website. Far be it from me to advise a marine surveyor regarding your electrolysis issue, but we had a similar problem and by accident discovered that a sharp edge of our copper foil grounding strap had chafed a hole in the insulation on one of our power cables under and behind the engine. Replacing the cable and covering it with shielding solved our problem although that exercise was a serious PITA!!!
No worries. Surveyor's don't know everything...and I say that at tremendous risk of being banished from the brotherhood!
I often consult with experts in various disciplines, and am consulted often by experts for my opinions.
Electrolysis can be such a far and wide ranging issue. However, I'm beginning to suspect the root of our problem may be similar to yours. We removed the SSB installation from Jo Beth when we bought her; both tuner and radio were old and tired and we really didn't need it as we wouldn't be ranging far offshore for a while. While we removed miles of unused cabling from the prior owner's various radio installs, we decided to leave the copper strip intact. You know, should we decide to put SSB back on board. We repatriated the breaker for the old SSB circuit to another use.
Long story short, we do have several places where the copper strapping and wiring are in contact, along with some tears in the foil itself. With the former SSB breaker now handling our 12vDC outlets, well...Plus, we have an incomplete bonding system. Add to that, that I still think there's an issue with our marina docks as dozens of boats on different docks have identical issues. I'm not above installing a galvanic isolator in the boat, but if the problem be us, that won't be a guaranteed fix.
I'm glad to see you got your issue resolved. We'll see on our end. right now, I'm well stocked in zinc collars and have a diver on an every 60 day cycle to be sure we stay ahead of it until it's fixed.