I couldn't justify buying one ($300+ for a good one) and instead borrowed one from a friend who is a surveyer. You might want to ask a surveyer and accompany him to draw a map and take pictures. I think it's cheaper than buying a meter.
Alternately, see if a group of boat owners near you is interested in buying one together. The one I borrowed is an old JRC model with an analog meter. It's not the absolute reading you go by, but the relative changes. Particularly when there is an abrupt change, you need to investigate. It has been accurate in every place that showed moisture. It will not show where the wet areas are rotten core, or just wet. You have to take samples for that.