moisture readings are suspect a best. One of the problems is that of adjacent metals (outboard brackets), and bottom paint. both of which will give false readings.
What moisture meter are you using, and what relative % readings are you getting?
Also the old fashing "sounding" method (using a phenolic hammer or screwdriver handle) is tried and true. We have done some extensive ultrasonic testing, and that is the best--but not available to the public. Also FLIR (infrared) photo assay is very good. A portable IR unit, rather than a full camera (Ie the FLIR Scout, or the I phone adaptor) can give relative readings, which may be of value. However, again the metal and bottom paint will show different readings and at times can be an issue with interpretation.
Put the motor in full trailering position. Put your weight on the lower unit just above the prop and bounce a couple of times. If the transom flexes, then it is suspect.
I know of several transoms which had to be replaced. One appeared to be sound in a newer boat, and when a log was hit, it caused cracking of the transom, and it had to be re-cored.
Even in the 25 that I owned (and documented in my album) where the transom was initially suspect, turned out (by drilling as well as other methods) to be solid and no issues in the long run. This was a boat which had been abused, and known issue with the cockpit deck which we replaced.
Tell us a bit more about the boat.
