really depends on the resins used and the quality of the lay-up in the first place, plus any treatments to the hull
. Temperature is also a factor.
With a 20 year old hull, it is unlikely to be a cored hull, as most back then were a solid core
. Therefore th worst that is likely to have happened will be osmosis
- and that will be immediately evident as soon as the boat is taken out of the water
and cleaned off.
No boat has ever sunk due to osmosis!
However, I would be very suspicious of through hulls and especially any metalwork underwater. If she has really been in water
for 20 years, then the zinc's will not have been renewed, and any metalwork underwater is very suspect, that also includes propellors, rudder
fittings and through hulls - these can sink a vessel and have done so many times.