To try to get back on topic, how much error is there in an amateur's use of a moisture meter to look at boats, and what are there the limits of how much you can learn with a moisture meter about a boat that is in the water?
Error depends primarily on your experience with the meter and knowledge of material (isn't everything?). Someone who just bought whatever meter and went to use it on a first boat likely won't derive any useful information whatsoever. Someone who had used moisture meter consistently (and took time to physically inspect materials he tests) - would probably be as good (or better, as my last survey had shown
) as anyone. It's not magic - but it does require some knowledge and experience.
You can always check the deck of a boat in the water. With cored deck and non-cored hull construction, deck is where moisture meter most useful anyway.
A good moisture meter should also be designed to ignore "surface moisture" and take readings at a small depth (to ignore any mist or run-off), and have atmospheric moisture adjustment (without that you will have different readings of the same surface on humid or dry days). Appropriate equipment is a significant part of success.