In my experience it is almost always a boat using incorrect AC & DC wiring practices. It can be marina or shore power related cause but this is much, much less common than boat owners, builders or service techs who really don't understand marine AC & DC wiring.

A few links on ESD:

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-explained.asp

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-tech-talk.asp

http://www.electricshockdrowning.org/