It can get rained on one time, and if water finds its way into the wood thru a bad seal or screw hole, it will rot the wood. The water needs to get to the bare wood to rot it.

If it is all ready rotten and then was stored inside or out of the weather for a while, Moisture meter will show dry, but damage is all ready done.

I had an 88 Donzi Z33..rotted stringers, bulkheads, transom, and floor, in fact I put my knee thru the floor. There were no weep holes anywhere so water trapped, then the cockpit drains had no tubes to direct it back to lots of water sat for years. There is only one way to know and that is a core sample, a hammer works but a drill tells the truth. On this boat they notched a stringer bad for the companion way and a drain, it was only 3"s high there and cracked in half. after an entire summer and winter of work it was better than new, we doubled the stringers and bulkheads, basically stripped the boat short of splitting the deck. It was a mess and taught me a huge lesson. My Active Thunder was solid but also a 94. Most of these older boats are not taken care of and you can tell right away. Here is the Red Menace when done.

Last time I was at the fiberglass shop, there was a 2000+ Larson in there with hull delamination. Last year he had a Crownline in there that was totally rotten, totally done. Guy bought it without getting a survey. Seems like every time i'm in there they're working on some kind of boat.

On my '91 Fountain, I have my fiberglass guy check it out with his moisture meter at least annually, to see if there's any evidence of moisture getting in, so it can be dealt with before it turns into rot. So far, so good. I did go so far as to move the mounts for my tab indicators off the transom (now attached to the trim tab ram itself), and move the hull penetrations for the hydraulic steering above the waterline. That stuff never should have been mounted to the transom below the waterline to begin with.

I know a guy with an older Nova that had a little rot in his transom. The moisture in the transom caused the bolts in the transom housing to corrode and one eventually broke, letting in lots of water and nearly sinking the boat. The transom was only wet / rotten for about 6" around the outdrives, and only below the waterline. He fixed it with git-rot.