composite (plastic) through-hulls are a fairly common problem, as a walk
in pretty much any boatyard will bear out. Ultraviolet light is
the main culprit here and while different brands vary widely in their
susceptibility to UV damage, some are so poorly made they can fail
within a year. Although manufacturers began adding inhibitors to
second and third-generation fittings to make them more UV-resistant, the
older ones are still around and it's almost impossible to tell
one from the other (one reason I view all of them with suspicion).
Composite through-hulls typically fail where the body of the fitting joins the
outer flange, resulting in a crack or total failure (both of which are
shown in the accompanying picture). Once the flange is sheared
off, there's nothing left to keep the through-hull in place, meaning
it'll be pulled inboard, leaving an open hole in the hull. Such
failures at or below the waterline can easily result in sinking, however
even those well above the waterline can be just as bad - as in the case
of a bilge pump discharge, which now just continues to pump water back
into the vessel, meaning the waterline rises till it reaches the hole
and then...well, you get the picture.
Plastic fittings should be inspected at least annually. If you find one fitting that's bad, play it
safe and replace the remaining ones too; they're probably the same age
and prone to failure as well.