I am definately no expert (ark my wife)!!!! but my personal experience would suggest that you could try the following which might alleviate the problem. Check that the bonding straps between engine parts and/or sacrificial anodes are properly connected and not inadvertantly "insulated" by paint or grease. Also check that the sacrificial anodes themselves have not been "protected" by paint, grease, wax etc, especially the one at the base of the engine bracket.
A separate sacrificial anode fixed as low as possible ( must be under the water when the boat is at rest) on the outside of the transom and connected directly to the battery earth terminal has also been recommended to me as a solution for reducing electrolitic/galvanic corrosion and I have seen it installed on a number of boats.
I hope some expert can confirm this "possible fix" as being appropriate for your problem.
Finally, switching off the battery isolation switch has been advised to me as another possible "fix". Good luck.