Some one said 1/4 inch of FRP (to 80's standards of glass/mat/resin) and blue water all in the same sentence.
Even without knowing the history of the boat, flex history of the hull etcc. That's Living life on the edge.
My little tiny EU Open Ocean Category A rated Gemini is built better than that, and while other's have I would not take it open ocean.
Island Packets (good rep Blue water boats) have 1.5 inches at the keel of glass alone: http://www.sailmag.com/features/IslandPacketFactory.pdf
Catalina 30's, a good coastal boat - have 1/4 - 1/2 (varies depending on how hungover the factory worker was that day) of FRP down low.
My seriously coastal only Hunter 31 had 1/4 of FRP - when the liner was pulled to replace it we could see sunlight glowing thru it, hence the move to a Gemini even for me.
Equally important is how the hull is braced with framing, bulkhead and stringers, and how the deck is bonded. Th simple fact of the matter is the Irwin is a splendidly designed low cost charter boat designed for coastal and near coastal use. Not for crossing oceans. Can it be done? Certainly, with luck and big assed pumps on board and a crew to man them and caulk seams and do damage control.
I could sail my 10.5 ft 1/4in plywood, glass coated dinghy to England tomorrow to. Maybe I'll start a blog and get some donations rolling in.
Not for me, for my soon to be widow.