Electric water heaters in my experience are the number one cause I've found.
Number two? Old battery chargers where the internal insulation breaks down and the shore power side of the unit starts to leak over to the DC side trying to charge your batteries.
My number three most common cause is a neutral to ground bond at a point in the boat's electrical circuitry other than a source of AC power. These are most often created by land based electricians doing what they have been trained to do in land based electrical installations. This should never be done on a boat.