There was a recent article in one of the boating magazines pertaining to the use of various core materials, with balsa listed as the best wood because of its structural integrity being higher than marine plywood. The same article talked about teak V/S some of the new composite, faux-woods such as Trex, and others currently used for decking material. The biggest problem with any of this is the change in weight distribution over what the boat was originally designed for.
A number of small boat manufacturers, such as Boston Whaler, use a dense form of foam that injected between the inner and outer hulls. The foam is very adhesive, and delamination is extremely rare. For transoms, nearly all powerboat manufacturers have switched to using a honeycomb, fiberglass core that is incredibly strong, it is tightly adhered to the inner and outer hulls and weighs next to nothing. The beauty of this approach is that when screws, through-hulls, etc.. are inserted through the hull(s) if they spring a leak it's confined to one, small chamber and does not effect the rest of the hull.