A moisture meter is a great psychological tool in addition to the data it can actually provide. A moisture meter is like a lie detector- you don't have to know how to use it, just having it visible can often achieve some truth and a lower price. Whipping out a screwdriver and a hammer, on the other hand, can really get a seller's back up.
Boatpoker, this is one time where i will disagree with you as an average boat-buying schmuck. You, as a surveyor, can percussively test a boat without a seller getting too uptight, because, since you have introduced yourself as a surveyor and presented your creds, there is an implication that you know what you are doing. If me, a boat buying schmuck, decides to start sounding a hull a seller is likely going to throw a fit.