The question I would have is ... "does the meter provide an accurate reading?". Pinless meters are tricky, and this Ryobi meter is a relatively cheap meter. Yet, the reviews by a few people who appear to be relatively knowledgeable, seem to support it. Did you have it set for softwood? I'm pretty sure that is how I would set it. What is the core?
It's hard to know how the meter is affected by reading wood through a GRP layer. It's also hard to know how metal in the vicinity under the deck may be affecting the reading. I'm not sure I would trust the 0% reading, as it is rare to find wood that has no moisture, but kiln dried and laid up in a GRP wrap? ... maybe. The meter isn't accurate below 5% anyway. The 30% reading is definitely suspicious. To really be sure, a meter with pins is the best way to verify. That means small holes in the deck, if you can find a discreet place to do a test. I'd still start with the tap test to see if you can detect a different sound. That's non-invasive.
I think that if the deck feels solid, the tap test provides no evidence, and the suspected area of wet deck is not in a location where structural problems are likely to arise, I would proceed as Carl suggests. Keep in mind that the surveyor may have the same reliability issue with his pinless meter, if he is looking for wet decks. He may come to the same conclusion, but his opinion will probably carry some weight and be a point of negotiation.
That looks like a fine boat and worthy of the serious consideration. I wouldn't walk away without knowing for sure if the decks are a serious problem.