Other than contacting the manufacturer or other owners ( or even local
fiberglass/boat repair places who may have worked on that make of boat.)...
the other way ( still risky) is the "sneak up" method.
It is:

You drill a small hole ( maybe a 1/4 inch). You stick a bent piece of wire
down the hole and make a circle with the wire. If you don't hit anything,
then your "safe" area is double the length of the bent wire. You drill a
slightly larger hole, maybe 3/4 of an inch. You stick in a longer piece of
wire...make the same circle, and note if you hit anything. If you are
fortunate, you can scribe a hole large enough to put your deck port in. If
you do hit something, you may be able to move your deck port position away
from the under-deck obstruction.
regards, RichG TX
--
RichG manager, Carolina Skiff Owners Group on MSN
http://groups.msn.com/CarolinaSkiffOwners
..