I'm in croatia and my boat stays in the drink the whole year (since 2005), except short periods every spring to do maintenance work.
There is neither wet grp nor osmosis.
The humididy metering is a kind of hocus pocus some guys are doing. The problem of soaked grp dates back in times when laminates and resins were of poorer quality and the manufacturing processes lacking quality assurance.
As symphony said, all newer production boats have sufficient quality grp to remain in water all the year, if the outer skin (gelcoat) has not been destroyed. Some yards are suggesting expensive epoxy treatments of the hull (with sanding down the original gelcoat), drying out and applying a lot of layers of epoxy, which is in my opinion only neccesary for the old rotten bodies which are prone for osmosis (and are sometimes praised by their owners for their longevity and strength).
I had the same concerns about humidity of grp, hence bought a humidity meter and traced the humidity in my hull over 6 weeks on the hard with no relevant change over time.
The only wet spot is (and has been) the rudder, but this is a different story and it makes no difference neither wintering on land nor in water