Balsa schmalsa, so what? Foam rots too. If you have
water incursion in your
core, it's not a good thing no matter what the core is made of. Even solid fibreglass has voids which wick moisture, causing
delamination.
OP:
Go here, this is helpful. This fella covers a lot of what you need to look for initially.
How To Do your Own Marine Survey by a Marine Surveyor
It's not a terribly bad thing that the boat's been
on the hard, unless it was stored badly, so look at the supported areas - if they're not at bulkheads, you might see indentations from the weight pressure. Look for big stress cracks at beams etc and know the difference between a serious stress crack and cosmetic fractures.
Bring binocs and look at the
rigging aloft as best you can. I've seen where the
rigging looked good at
deck level and had broken strands at the
mast.
Stick your
head in every little cranny, and don't be afraid to poke and prod at things. Pull dipsticks and look at
engine fluids. Pull up the floorboards and look in the bilges. Look at the
electrical systems. Look for rusty weeping at bolts,
backing plates, swages, chainplates. Ask to have things turned on that should be able to turn on, given the
boat is on the hard obviously you can't crank the engines. Look at the tankage. Look at through-hulls and fittings.
Best
advice I can give: even if you have to fly him in, get the best possible
surveyor you can. A few hundred or even a grand or so extra in travel costs could save you much more in trouble and cost