I agree with Slosail’s advice. But to understand why shear strength is the most important property you need to look at the structural theory of sandwich panels. For many boats and aircraft the most important failure mode is elastic instability – buckling or “oil canning”. A skin of very thin carbon can handle the in plane stresses and strains but the structural stability is dependant on panel stiffness. The core material plays the same role as the web of a steel I-beam to separate and rigidly attach two load bearing faces. So, this depends primarily on the shear strength of the core and the strength of its attachment to the face laminates. But boats are also subjected to all kinds of off-design loading including elbows and knees impacting with lots of force (I have personal experience here!). So, cores also have to be damage tolerant in the sailing environment.
By far the most efficient core from a structural is honeycomb but this is very expensive. Balsa is heavy but very damage tolerant – it will rot if given a chance. I use balsa in high load areas and structural reinforcements. There is another type of foam not mentioned under the trade name “Rohacell”. This foam has higher properties than the boatbuilding foams and can be processed in an autoclave at 130 Deg. C and 30 PSI. Unfortunately is expensive but not quite a much as honeycomb.
Early specifications for aircraft sandwich panels called for dropping a 1 inch steel ball bearing from a height of 40 feet onto test panels and then examining for any local delamination of the face/core bond. Perhaps not a bad approach for boat decks?