Thanks Bob, that is some serious bed-time reading there! Good link.
For my part (assuming it is osmosis) I don’t think the cause has been water inside the boat as it was been kept dry by and large at least since I have owned it and I suspect it has always been a well-cared for boat. I suspect the more likely cause has been keeping a 30 yr old boat on fresh water for over two weeks at a time the past two summers without waxing the hull (don’t tell anyone). Pragmatically I have waxed it after drying out thoroughly and will see what happens this year. Just as well I don’t own a yacht………that looks a horrible problem to sort.