I think both of you are spot on. The bubble sounds like an spot where the veneers didn't get glued together. Charlie is right that you have a legimate beef with the seller. But since the wood is already in your boat, a replacement piece won't do you much good. The plan to inject epoxy into the void should work well and will not result is an inferior boat. If I had to do it I might slit the top veneer with a utility knife, raise one edge, force in unthickened epoxy, do the same to the other side, cover with waxed paper, and weight it over night. Give it a light sanding, glass over it and get you boat out on the water.