In very general terms, boats built prior to the first gas
crisis tend to be more likely to be "solid" glass than
those built after. ...

Great tip! Then I should be happy to hear that a boat is from the
1960's or the early 1970's. Good! Those old boats should cost less
anyway.

... BUT, in the smaller boat (and some larger ones too) you
can also run into some built by "chopper" gun, i.e. resin
and fiber sprayed into a mold instead of layers of woven
cloth or mat and resin.

If a small boats were built by "chopped" gun, its hull material is
still made from fiberglass, and not those "cored material", and should
be good in term of limiting water damage, right? I am under the
impression that the problem of a boat that were built using "chopped"
gun is that it is not as structurally strong as a fiberglass boat that
was made from long-thread fiberglass. I am wondering whether I can
strengthen such a boat by wrapping the hull with two layer of
fiberglass.

Obviously, this kind of boat should not be my first choice. I am asking
this just to satisfy my curiosity.

There have been so many manufacturers who have come and gone, some
producing both good and bad boats throughout their lifetime. You will
probably have more luck getting information/knowledge if you dug up a
couple of "sample" boats and began your questioning/working from there,
lord knows there are a zillion 18 ft runabouts lying in backyards and
back lots, many there for the taking.

I see. I will have to shop around and then get back here.

As for wood? If you find an old wood boat that has been coated very
late in its life with glass and resin, it's probably a disaster in rot,
the same boat, if it was built piece by piece and epoxy encapsulated as
part of the construction process, can be both beautiful and strong.

Thanks for the tip. I will watch out for this.

..., you CAN be reasonably secure if you look at older
Boston Whalers. They have always been well built, are
very tough and hard to damage. Some older ones may
have absorbed water in between their two hulls, but
that can be dealt with by hanging them from a tree
for a year or two, with a couple of points allowing
the water to drain slowly out, generally.

Thanks for the suggestion. But seem like this kind of boat has the
interior of the hull totally sealed up by deck, and cannot be easily
examined and maintained. I guess I could cut away the "fixed deck" of a
Boston Whaler and convert it into removable panels. But I am afraid
that the "fixed deck" may be a part of the structure. Cutting away the
"fixed deck" may weaken the boat. I am just hoping that I can find a
boat that has removable deck panels that we can move away and examine
the inside of the hull.

Jay Chan