You make several references to a "fiberglass boat" in salt water. There is
nothing about the hull of the boat that is made any differently for salt
water or fresh. It isn't the hull you need to worry about, it is all the
metal parts.

Salt water causes metal to corrode/rust much faster. Many items you can
ignore, others you would be better off to pay attention to.

The obvious areas to focus on are those areas that get wet. The cooling
system of the engine is one such item. A boat that was built for use in salt
water would have a closed (a.k.a. fresh water) cooling system, where only
fresh water/antifreeze circulated through the engine and the sea water was
used to cool the heat exchanger (the equivalent of the radiator). For just
an occasional trip to salt water you don't need to be overly concerned, but
it would help avoid problems later on down the road if you hooked up a hose
and flushed the engine with fresh water as soon as possible after running in
salt water.

If the boat was going to be left in salt water for more than a day at a time
then you would really need to pay attention to your anodes and general
maintenance on the outdrive. Salt water can really accelerate electrolysis
which can eat away at aluminum. If the zinc anodes are in good shape you
can minimize such damage.

Rod