If the boat is wet slipped it is necessary. If trailered it's a moot issue.
The system if operating will draw power from your batteries constantly. Keep an eye on them so they don't go dead.
http://lmgtfy.com/?q=how+to+test+a+Mercury+Marine+Mercathode+System
If your boat is equipped with a Mercathode system, there is a simple way to test if the system is operating properly. The goal is to check the output from the Mercathode controller. When the boat is in the water, disconnect the orange wire that comes from the anode at the mercathode controller. Set the digital reader to show milliamps. Connect the black meter lead to the terminal on the controller. Connect the red meter lead to the end of orange wire. If using the blue or black Mercathode controller, the reading should be 25 or less milliamps, to as much as 200 milliamps in saltwater areas. If using the red Mercathode controller, the readings should be less than 25 milliamps in fresh water areas to as high as 400 milliamps in saltwater areas.
If the boat is not in the water you should also check the condition of the cathode wire on the outside of the boat. This is located on the bottom of a box mounted at the bottom of the transom ring. If you lay on your back under the drive (in trailer position) and look up at the bottom of the transom ring, you will see a bare wire running across the bottom. That needs to be clean (no paint) and not broken. If it is dirty clean it with solvent and q-tips. Sometimes these get bottom painted and then they don't work so good.
[PDF] [h=2]
SERVICE MANUAL NUMBER 14 CORROSION PROTECTION …[/h]
www.boatfix.com/merc/Servmanl/14/14a7.pdf
