I'd recommend you read the article I wrote on Cored Laminates
, as it should give you a pretty good overview about core materials and such.
Properly designed and maintained, a cored-laminate construction boat is going to be stiffer, lighter and stronger than a solid-fiberglass boat. Fiberglass is a relatively flexible material, and to give it the rigidity it needs either requires laying it up fairly heavily or adding a grid of stringers and floor or installing a pan or interior liner of some sort. Many cored laminate constructed boats can do away with the need for a liner or interior grid, since the more rigid cored construction allows them to be stiff enough to not require it. This is quite often the case in many of the one-off or home-built multihulls.
Certain materials have distinct advantages over others.
Balsa is the strongest of the commonly used core materials in compression and tension, but has the disadvantage of rotting if exposed to water. It will tend to prevent water from migrating long distances and delaminating large areas, at least until it rots.
PVC foams can be broken down into two categories—rigid foams like Divinylcell or ductile ones like Airex. All PVC foams have the advantage of not rotting, but can allow small amounts of water to delaminate fairly large areas by migrating along the interface between the foam core and the laminate skin.
The ductile foams are better suited for use below the waterline IMHO. The rigid foams are excellent for the deck and cabintop. The weight of the foam can be varied depending on the requirements of the foam cored laminate's purpose. You might use a lighter foam for making locker covers than you would for the hull or deck.
The reason I like the ductile foams for below the waterline is that they can provide a hull with greater impact resistance, especially when coupled with a kevlar or spectra laminate skin. Unlike a rigid foam, the ductile foams will tend to compress under impact, rather than transmitting most of the force through the laminate to the inner skin. This combined with a kevlar/spectra laminate skin, which is far more tear resistant than fiberglass or carbon fiber-based laminates—often results in the inner skin maintaining its integrity in an impact, and preventing water intrusion into the boat from a collision.
Some early manufacturers used marine plywood as a core material. Plywood has several major problems as a core material. First, it is heavy compared to balsa or foam. Second, it has the worst characteristics of both foam and balsa—it rots like balsa, and it can allow water to migrate and delaminate large areas of the hull relatively quickly like a foam core can.