This is what a failed zinc anode looks like when used in brackish water. The hard coating on the surface caused it to become inactive and ineffective.
Aluminum anodes do not passivate in brackish water and offer superior self-cleaning performance compared to zinc. They are the best choice for boats docked/moored in brackish water.
Photo courtesy of Scott H., Toms River, NJ ©2018
All rights reserved. Used with permission.