No. 1 insurance claim for pontoon boat owners:

galvanic corrosion.

It's an electrochemical process that can occur when different areas of the same metal are immersed in water, which is an electrolyte that is poorly conductive in fresh water, highly conductive in saltwater. The pontoon creates a metallic pathway linking sections of the metal, causing electrical current to flow and corroding parts of the pontoon. Most boat insurance policies don't cover this type of damage. Aluminum sacrificial anodes are best because they outlast other alloys.

Source: BoatU.S. News and Key-to-Steel.com