I went through weeks of this on a friend's boat. His "target" was about 1Mohm or greater from any electrical point to the hull. What really matters is current, not voltage. He had an installed milli-amp meter that measured mA of current flowing between various testable points.

We were disconnecting electrical "consumers" and grounds one at a time and watching how much, or if, the current changed. The last troublesome bugger turned out to be a shield ground on a coaxial antenna cable connector.

There is a plethora of information on this all over the interwebs on this subject as well as some great educational material. I think you pretty much have to become your own 'expert' as boats are so different. Crank up the Google.......