A boat isn't going to get a wet core from just sitting in the water. Osmosis isn't going to penetrate the laminate through to the core. It is possible that huge blister, left untreated, might cause a problem, but you wouldn't have to strip the bottom to see that.
If the core is wet, the water is getting in somewhere. For the hull on a J30, it's almost certain to be the exhaust and bilge outlets or the gudgeon bolts. If you have a sound transom, then you're probably fine.
If there is core damage elsewhere, it is likely the result of damage from grounding (around the keel), a collision or dock damage at the hull/deck joint, or the result of poorly installed hardware (like a swim ladder bracket for example).
Percussion testing can tell you a lot about the quality of the core and, unless you know how to really use one, can even be more informative than a moisture meter.
If you are dealing with wet core in an area larger than, say, a few square feet, you're going to make a significant investment in the repair: in time and money.
In short, the boat's age and the fact that it's been wet stored is not, by itself, a cause for concern. The poor effort by
J/Boats to protect the core and the subsequent years that have created opportunities for owners to drill a lot of unisolated holes in the boat are much bigger causes of concern.
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