Follow Amanda's advice and check out Barb's site. The large opening is fully open so the entire veranda isn't enclosed, but it is more enclosed (and to me more cozy) the a regular veranda will the open top and plexiglass bottom.
We've had both and I prefer the Navigator's veranda. It is more private, but you still get plenty of air and a beautiful view of the water. You also get a nice padded bench that is great for stretching out on and reading a book.
Some people prefer the regular plexiglass verandas so they can sit and look down at the sea or see the sea from their room.
That's what is so great about Disney - the choices you get!