Re: Balsa Wood?
The thing is, you can't order sheets of balsa to replace that in a cored hull.
The balsa used is "end grain" balsa, sort of like lots of little slices off a balsa log,arranged like a checkerboard.
If you're going to replace it the "right" way, you need to order some similar end grain balsa from somewhere specialized that sells it for use as a core material in boats.
Or, you could replace it with divinicell or renicell, which are foam cores that serve the same purpose and won't rot (but are just as hard to dry out).
Or if you don't care about the weight, plywood will work fine, or even making it a solid glass panel, although both of those are heavier than balsa or foam.
I'm guessing this is a foredeck section? If you can, work from the non-visible side of the panel to avoid a lot of extra work.
On the other hand if you're just trying to replace an ordinary plywood floor, read up on the various types of plywood in other posts here
Erik