Paulo : I think I would believe studies done by independent labs and Standards Societies rather than sales blurbs from the manufacturers of these products
I confess that I had only an idea that Epoxy was waterproof but after a search on internet I have no doubt that there are Hygroscopic and non-hygroscopic epoxies. The industrial uses of non-hygroscopic Epoxies (besides marine) are incredibly large: From computer chips to electric engines and building materials.
620 Underfill Epoxy
Hatteras NC1000 Drives - Tide Tamer
I guess that the confusion here is between composite materials with epoxy resin (that are hygroscopic to some extent) and epoxy resin, that can be non-hygroscopic. The sealant that is applied in several coats to the hull bottom is an epoxy resin and it is an waterproof one. All manufacturers state that and they would be sued if that was not true.