The submerged metallic parts of the yacht are protected against galvanic corrosions by means of anodes.
Check for their wear very frequently, as it depends also (and highly) on environmental factors like sea chains nearby, metal posts or docks, metal hulls moored nearby, electric devices, etc..
The replacement is necessary when the wear exceeds 50%.
Each time the yacht is lifted, check for the condition of the propeller, of the protection anodes and of the fastening system.
Replace the anode frequently.