Have a corrosion survey done or talk to a ABYC Marine Corrosion Technician. You can find one in your are via this link.
American Boat and Yacht Council (ABYC): Education and Certification - Certified Technician Search
We had a corrosion survey done this past July and it was well worth the $65 we paid for one hour. We've put Hobo in a marina, that's up a river, in brackish water and the summer rains hadn't really started yet. We wanted to see if our bonding system was in good order and that we didn't have any stray current in AC or DC systems.
The results showed that the bonding system and AC/DC systems were fine and we were within the recommended range of -550 to -1100 millivolt. We tested -635 millivolts. The technician suggested that even adding more zincs may not get us more negative because of the freshwater content potently increasing so he suggested that we switch to aluminum anodes. A week later after changing from zinc to aluminum anodes he retested. The reading was -1028. We changed out a 6" x12" x 1/2" plate, prop and rudder zinc all to aluminum. The change-over was ~$10 more.