There are several possible routes on this. First, how much do you want to spend on a '75 Islander 28? The original material delaminated due to leakage at the ports. Have you fixed the leaking ports? It is also notoriously difficult to stick plywood to fiberglass. The fiberglass keeps moisture in the wood from drying out helping to promote rot, and the plywood swells and shrinks as the moisture comes and goes, and the bond ends up breaking. Plywood is often glued and screwed to fiberglass in order to keep the wood in place when the glue joint fails. With this in mind, one possible route could be to use panel or tile adhesive, spread with a notched trowel, and some screws for good measure. Even if it doesn't last long, it's cheap, and would be easily fixed if necessary. A longer-lasting bond would be likely with epoxy. You probably would want to thicken it up a touch so it doesn't drip all over everywhere and need to be sanded off other unintended surfaces. Getting the pieces in the exact right places and holding them until the epoxy sets will be like juggling twelve plates on sticks. When the bond lets go, it will likely fail in one place, but not another. This will make replacing the plywood an intriguing exercise. How good are you at scarfing interior joinery... in place? Ripping out the sections that are still stuck (to avoid having to cut scarfs) may damage the fiberglass underlaying it. To make it less likely that the plywood swells and shrinks due to moisture, breaking the glue bond, you can encapsulate it in epoxy first. This gets expensive. You have to do every side of every piece, especially any endgrain. Two coats (perhaps three on the ends) should do it. Make sure you're working in a low humidity space, and that the plywood is nice and dry too. Moisture outgassing from the plywood can cause haze in the epoxy which has to be sanded out. Sanding epoxy is not fun. Don't ask why I know this. When your pieces are all encapsulated, you can use epoxy (thickened) to glue them in place. If you trim anything for a better fit, don't forget to re-encapsulate where you cut. So, there you have three options. There are probably lots of others. Among them would be 3m5200, 3m 4200, sikaflex... you get the idea. Good luck.