I agree with Chuck on grounding. Only an idiot would NOT ground the AC
systems to other items on the boat. As an example suppose your hot
water heater developed an AC short to its case. You are standing in the
bilge hanging across the engine to reach the thing to work on it. The
engine is at ground via the shaft etc. If the water heater is not
bonded to the engine ground you get zapped!
This is only one example of what will happen if you don't have things
tied together on a boat. It is next to impossible to insure that there
will be no AC leakage in any piece of equipment especially in the
marine environment.

Even the boats that try to maintain isolated underwater (thru hull
etc.) items are a risk. As long as the items maintain isolation it
works ok but as soon as some worker comes aboard and "fixes something"
the chances of the isolation remaining drops.

The only cure for the galvanic problem is an isolator or better yet the
isolation transformer.

Testing to see if you have any faults with an ohmmeter to the marinas
ground pin on the shore cable is futile. There will usually be several
millivolts of voltage above ground on the shore power ground pin and
some DC offset voltage that will immediately invalidate your ohmmeter
reading.

Measuring for DC current between the shore ground and the boat ground
will tell you if there is a galvanic problem.

Regards
Gary