Hi everyone, thanks for all the replies/suggestions. Some responses below...
Is another electrical item connected to the bonding circuit, other than at the engine block? That is a possibility and the bonding system shouldn't be used for a negative return. I would check this.
As far as the battery charger I guess its DC negative is grounded to its chassis....
...How is the battery charger wired as far as DC output?
What I said about the bonding circuit is possible. As far as the charger goes unless the negative output goes somewhere other than the negative bus or the batteries I don't see how that could complete the circuit.
The AC ground should only be connected to the DC negative in one location so that should have nothing to do with it.
Wish I was there - an interesting puzzle.
mitempo, nothing that I KNOW of is connected to the bonding circuit. That doesn't mean it's not there though. Could be something a PO did years ago that I wasn't aware of and haven't yet discovered.
The charger is connected straight to the DC neg bus. AC side connected straight to the AC panel on its own switch. AC ground bus (on back of panel) is connected to engine block. As far as I KNOW, no breaks or weirdness there.
I do have a few theories, but I'll need to go back to the boat today to try them out.
And speaking of interesting puzzle, I'm still confused as to how the entire boat would be powered up. If something was tied into the bonding circuit for instance, it would need to also be tied into the DC negative. Possible, but I haven't seen it.
If you have a solar panel wired in it's got a ground connection ????
No solar, but thanks for the suggestion!
Bet it's the battery charger!
Now, if you remove the negative cable from the battery and your boat still lights up, then you've REALLY got a problem :-)
That's kind of how it felt when I first found the problem.
I'll head back to the boat today and disconnect a few more things in sequence to see if I can further isolate the issue. I can disconnect the bonding separately at the block, for instance. I'll report back.