Steve wrote:

Is there anyway, with simple equipment that I can determine if his boat is
causing any problems? Should I test between the neutral lead of shore power
and the water for stray current with a volt meter??

Steve,
what I would do is simply measure with an AC voltmeter (possibly digital
and with high internal resistance) if there is voltage between the
ground wire of the shore power and the water around your boat when you
are NOT connected to shore power itself while the possible suspect IS
connected. Useless to say to be very careful. Try different scales
starting from the highest down. Eventually, if the voltage is low, try
also the DC scales. You should not be able to read any voltage but if
there is a stray current problem you might. If you read anything let me
know and I may help to work things out.

Daniele