A moisture meter pegs at a pretty low level. I bought a 1979 J-30 with a significant wet core problem. The moisture meter I have (Sovereign Skipper) pegs pretty much everywhere on the starboard side of the hull, but the core samples I pulled above the waterline were light and fairly dry. Lower on the hull there was significant moisture.
The best test is to pull a sample and see how much moisture can be squeezed out in a vice. If the core is light in color and water doesn't squeeze out you probaby don't have a serious issue, if its dis-colored and water squeezes out you have a core problem and should negotiate the price accordingly. In any event you will need to locate the source and stop the water intrusion.
Also, the water will migrate down in the hull so take the core sample as low as possible.
