But I'm sitting here in a 100 y. old granite house and it's rained like crazy 3 days in the last 2 weeks. Not a leak in sight from the granite.
So, I would suspect that it is not the granite itself that's leaking but the joints between which can, in fact, be reworked (like brick joints - - repointing). So, sealing the surface of the granite would not solve your problem. Filling the joints has to be done here (Spain) every 10 years or so (in spots, not the whole house). A medium thick concrete compound is used to squeeze in between the granite blocks.