For those looking for a locally available solution, Plexiglas G (and only the "G") is water clear and works quite well, except for the fact that it is not optically coated. It is relatively soft and can be scratched, so always use a plexiglass cleaner on it. The surfaces are optically flat and will not cause any distortion in the screen image like you would find with Lexan.
Finding optically coated water clear glass in most cities is just about impossible without special ordering it. Finding optically coated water clear glass in a large sheet, 12" x 48" for example, is even more difficult.