If the engine was supposedly bonded and has 600K ohms of resistance to the ground lead on the shore power plug, then something is very much wrong. If you decide to go ahead with this boat, I'd suggest you get a galvanic isolator and put it just inside the boat past the shore power line, then run a brand new ground lead from the AC ground (green wire!) to the engine block or the 'star' point of the bonding system. Then I'd poke around with a digital voltmeter and see what kind of resistances you find between the engine block and the battery negative terminals. It should read very near zero. (Just make sure to pick a spot with no paint on the block!)
Since it's a gas engine, and gasoline has some nasty habits (like happily exploding in the presence of an arc or spark), you need to make sure that the Fuel fill fitting is electrically attached to the gas tank, and the gas tank is attached to the engine, and the engine is attached to a grounding plate that is in good contact with the ocean. The whole idea is to keep everything at the same electrical potential. I know this sounds crazy, but when you're filling up the gas tank, especially during the summer when it's reasonably dry, the gasoline rushing down the rubber hose can build a static charge. Then you move the fuel nozzle a bit, provide a tiny ground and you get an arc--boom! It happened to an older Tollycraft sport fisher down in Miami a couple of years ago, and the resultant explosion and fire burned about a quarter of the marina down, and a lot of adjacent boats!
Another thing to consider, if you're not too far along: check with your insurance broker. Ask him the difference in the cost between insuring a gas vs. a diesel boat. You'll probably be shocked.
BTW, I'm impressed that your surveyor checked things with a meter. Sounds like you got a good survey. Ditto on the moisture numbers.
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