It would seem to me that it would be easy to check out the water around your boat in a marina with a digital voltmeter, a metal rod with an insulated handle, and a few wires.
Simply attach one lead of the volt meter to the ground wire of the hot wire of the AC circuit, and the other to the metal rod, and read the voltage. (Being sure to NOT make yourself part of the circuit!)
(One would have to establish beforehand by testing the AC circuit, that the voltage was within the normal range, the ground circuit solid and good, and the neutral wire insulated from the ground.)
Any significant voltage present would be an indication to stay out of the water.
One might argue that no one should ever be swimming around a boat in a marina, but bottom cleaning, prop replacement, skeg and shaft work on inboards, marina repair, and many other needs dictate that some folks will be going into those waters.
It might be nice to set up a permanent testing arrangement, understanding that the electrode should only be put into the water for testing (to avoid permanent corrosion), and that the ground wire should not be attached to the ground circuit for safety's sake, least it provide a dangerous ground source should a charge be introduced into the water between testings.
I'll be testing the marina waters around by two boats at Shasta Lake soon!
Joe. :teeth :thup