It is really hard for me to imagine how this pressure test was done in a way that could prove or disprove anything. If tape was involved, even Gorilla tape would in my opinion hold maybe a couple of pounds of air pressure at most before blowing out. Water can and does migrate through the joints in the balsa, but it does so crazy slowly and directly through the balsa even slower. My balsa drying project last spring proved that to me and that was with 15 to 20" of Hg trying to suck it through. Around the shaft log is solid GRP, it can be wet from years of saturation, but but water ingress directly through it is going to be crazy slow.
Water from the cooler, maybe. The shaft and the logs and couplings seem way more likely to my inexperienced point of view. Or there is another source from the engine itself dripping water into the bilge. There are lots of hose joints between the inlet and exhaust that all need to be carefully inspected. Not to mention the elbows that die a slow death and could be dripping into the bilge and would not show up on even the best air pressure test.
