FS is absolutely right about balsa core! I think it's better than plywood for deck cores provided you use some common sense and don't put unsealed holes in it!
I've read a number of yacht surveyor Dave Pascoe's excellent articles, and this one
http://www.yachtsurvey.com/core_materials.htm indicates that he likes balsa core better than most core materials! I suspect Carl Moesly researched it quite thoroughly before he started using it, as he did with most anything involving boats! Water doesn't migrate quickly through it like plywood, it has better shear strength than most of the foam materials, you always know what you're getting with balsa core (unlike many foam materials), and resin bonds to it really well, which is not the case with most composite materials. It seems like they really require the use of cabosil or equivalent and maybe even vacuum bagging to be really confident about getting a good bond with fiberglass!
Every square inch of the balsa core in my 42 year old boat is just as solid as the day it was built, probably because the original owner, who only had it for 3 years, never drilled any holes in it! I've always used the drill-oversize-hole/backfill-with-epoxy-cabosil/redrill technique on any holes I put in it, so the stuff should last forever if it's properly sealed and maintained!