As in so many things, the devil is in the details. Quite literally, this is the glue that holds it all together. It is also a determining factor in whether a boat will develop osmotic blistering problems. Isophthalic polyesters are much better. The most cost-effective way to produce a light but strong boat is to use a cored laminate.
Synthetic foam is increasingly common, but end-grain balsa is still widely used.
Cores are almost always used in decks, but are also often employed in hulls, particularly above the waterline. Joints bonded by several layers of fiberglass are also quite strong. Joints fastened with self-tapping screws or, worse, pop rivets, are much more likely to cause problems over time. Also, make sure bulkheads are tabbed into the deck as well. Fasteners and fittings.