OK, Y'all, here's the physics. The fiberglass fibers would be nearly
clear if they were not small enough to scatter light. Imbedding them
in a matrix of resin that has an index of refraction close to that of
the glass reduces the scatter and makes it seem more transparent.
"Opaqueness" of some hulls is most likely due to excessive scatter from
impefections in the resin. Thus, you might expect a very well built
hull to be nearly transparent except for some tinting due to absorption
of color.
This makes me wonder if it might be possible to make a hull bottom
nearly transparent.