Balsa has also been used in modern boat building, it is sandwhiched between two skins of fiberglass, another variation of this system, and more common is replacing the balsa with a closed cell foam plastic. With the origional open cell foam, and the balsa, if the outer skin was holed the whole interior became water logged. I know a little of the closed cell foam construction as my brother in law built a 16m catamaran, and used it for ocean cruising, up to Fiji, and on to Austrailia.
The important part of the design is the two skins, the balsa, or foam is there to keep them separated. There is a metal equivalent in skins on aircraft with honey comb cell separatingthe skins making a strong but lighter structure.
Last airshow in NZ this weekend, for the Mosquito, just a bit north of Wellington, at Masterton, it will be flying with two Vampires, and a Venom, along with about seventy other aircraft. Ian S C