Like I said before problems with water in the balsa core would be shop related issues! Now to qualify above statement that’s assuming the resin and laminate schedule were even halfway in touch with reality.
We had a lot of gunboats on the gulf all of them with balsa core I never found a core related problem, That whole experience changed my outlook on balsa core.
Water intrusion from the inside then look to see if the last lamination was top coated with an inhibitor.
I have heard the weight gain/age thing before and it was traceable to the two-part foam around the tanks
Bottom pressure ??? What are you talking about in psi? , In “testing” I have cut thru the outside laminate exposing bare core making changes in the bottom geometry and never had a hydraulic/delaminating problem in those days we ran 115-120 these were Airex boats but water pressure is water pressure , That said on return to the shop for modification/race prep no evidence of any damage was to be found and that is pushing the point!
In the old days balsa came in blocks that you placed one by one into the wet glass one of Harry Schoells guys figured out the scrim system as legend has it..