Any time you have two different metals that are physically connected and immersed in seawater, they become a battery. Some amount of current flows between the two metals. The electrons that make up that current are supplied by one of the metals giving up bits of itself to the seawater. This process is called galvanic corrosion and if left unchecked it quickly destroys underwater metals.
One cannot overstate the importance of maintaining the anodes on your boat lift (and your boat). When an anode is missing or largely wasted away, the metal component it was installed to protect begins to dissolve — guaranteed. Zinc anodes should be replaced when about half of the anode has been lost to corrosion.