Every time this question comes up there is always someone who argues the strength advantages of balsa over foam which is completely irrelevant as the typical 80 -100 kg/m2 foams typically used in boat
construction is more than adequate. Even the old polyurethane
foam cores were perfectly adequate as long as the density was in that range. I have a section of foam cored cabin sole
that I keep around as an example of how well foam can survive being completely saturated and then allowed to dry out. It was taken from a 1977 sailboat and appears to me to be polyurethane
, zero decay, zero delam. Balsa would have been mush. That said there is nothing wrong with balsa if it can be kept dry, I have seen deck
areas that are not fastener intensive on 40 years old boats that are perfect.