Kind of a difficult question to answer but neither percussive sounding nor moisture meters will work on frozen structure. get a small hammer put some masking tape on it and get a neighbour with a relatively new boat to let you tap it to see what good core sounds like. Good core will produce a solid "click" and almost no bounce back of the hammer. Wet core will give a dull thump with little bounce back and core that has rotted and separated from the FRP will give a pronounced bounce back sort of like hitting a drum skin. It does not take long to get the hang of it if you listen carefully. There is no need to hit hard, just gentle taps letting the fall of the hammer do the work.
This is one of the few areas where a moisture meter will be quite accurate so
if you can borrow one it would take all of five minutes to map out the entire trunk.
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