Where should I look for corrosion?

Electricity always seeks a return path to its source. A typical circuit has a positive wiring running from the battery’s positive terminal to the vessel’s fuse panel, through the wiring to the appliance. The negative cable from the appliance runs to a bus bar, and back to the negative post on the boat’s battery. The circuit completes a circle from the battery to the appliance, and back to the battery. 

Stray Current corrosion would likely occur if within this circuit the wire is broken, a connector isn’t crimped properly, or a connector isn’t watertight and that wire runs through (or near) bilge water.

Most often, stray-current corrosion involves a faulty electrical connection that makes contact with a submerged metal. This is often seen when wire runs are near bilge water.

A wire running to an automatic float-operated bilge pump was crimped, but not watertight. Current runs from the connection through the bilge water to a metal through-hull fitting. Current continues to flow into the water around the boat, through the propeller and shaft, to the engine block, which is where a common ground exists, and runs back to the negative post of the battery. The circuit is complete, but through an unintentional route. 

Even with relatively low voltage 12V battery power, corrosion can occur on the propeller, shaft, sail drive or stern drive within hours or days.