Far better than a sniffer dog is a jar full of pregnant death watch beetles. These have a strong preference for rotten wood for laying their eggs, so they are simply released in the cabin and watch kept for where they head. No special training is required, and there are no problems about reaching overhead places, but it may be desirable to paint their carapaces fluorescent yellow to aid tracking.
Having death watch beetle aboard thereafter is a small price to pay for this eminently cost-effective means of identifying rotten spots in wooden boats. Local surveyors are investigating whether pregnant polyestermites could be similarly used to detect osmosis blisters in GRP.
I'm working on a similar theory involving Sodium Chloride; testing seems to show that a sufficient quantity can indicate where ferrous metals are prone to corrosion.