There are many causes of marine galvanic corrosion. So many, that one could write a book on the subject but today we will talk about stray current corrosion.
What is stray current corrosion, you may ask? One way to think of electrons is like rain water on the deck of your boat. If you watch the water it will start to flow towards the lowest point, because of gravity. When you move about your boat, you change its balance and, as your boat's balance changes, it tips in a new direction. Now the water will try to flow to the new lowest point. Like the rain water, electrons try to move to the lowest electrical potential. This flow of electrons is stray current. Stray currents, which can come from variety of sources, corrode metals. As the electrons flow, like rain water causing erosion, they cause metals to corrode. This is because corrosion is a chemical change in the metal, which requires electrons to move through the metal. Without a current flowing, there will be no corrosion, so anything done that can limit the stray current will reduce corrosion. A common source of stray current is though your shore power ground connection.
A simple, cost effective way to limit shore power stray current is with a galvanic isolator. A galvanic isolator is designed to block typical levels of stray current from entering your boat, but let dangerous levels of current flow safely to ground. PYS will only install "fail safe" galvanic isolators. Older and non-"fail safe" isolators had a tendency to burn out if overloaded, disconnecting the shore power ground wire. This effectively disables all ground fault protection on your boat. A "fail safe" galvanic isolator is slightly more expensive, but comes with the assurance that if something goes wrong with the isolator, it is designed to always maintain the ground connection.