Apparently there is even a waterproof cast cover that you can put over your cast. In any case, I can see their point on how water can run through it.
I trained with the traditional cast liner, cotton, and plaster cast. Then later, the fiberglass ones.
I guess I just have not realized the technology of goretex liner, and how waterproof it could be.
My reservation about scuba, is, as the pressure increases, water will seep between the goretex and the outer cast material. The cotton or synthetic liner then could get wet, get mouldy or harbor bacteria.
While this is not a concern if the goretex liner keep your skin completly dry, but if it allow for the overgrowth of bacteria to seep against the skin, then problem can arise.
My gut feeling is, this cast might be ok for swimming or snorkeling, but for scubadiving, you might take a little risk in getting the inside of the cast (not your skin) wet.