I would like to expound upon what Alan said in his first post on this thread. It is true that using glass to simulate water has been around for a long time. In fact, Bill McClanahan, in his book, Scenery for Model Railroads, first published in 1958, describes the process and notes that “This popular method of building a pond or lake has been used by such craftsman in the hobby as John Allen, Frank Ellison…” However, back then, the idea was to look through the semitransparent surface and see the lake bottom. In effect, the glass was suspended over a cut out with a completely modeled lake bottom.
So while the use of glass is not new, the technique of opaquely coating the underside is. This essentially allows the glass to act like a mirror when viewed from the proper angle. And just like in real life, when you peer down into the harbor it appears murky, but as you lower your angle of view, reflections come into play.
There is a significant difference in realism obtained between the two methods (transparency vs. reflectivity) and as other products such as casting resin became more economical, the use of glass diminished. When I was designing the Sandy Harbor, I ruled out using anything that you had to pour. This was mainly due to the relatively large size of the waterfront. I wanted something simple and I resurrected an article I had saved from a 1989 Railroad Model Craftsman by Phil Chiavera. He had used a friend’s discarded shower doors to create a harbor for his HO scale NYC West Side Freight Line. Other than painting the underside green, he supplied no specifics. An internet search yielded nothing, so I set out on a proof of concept.
Experimenting with various pattern glass samples obtained from a glass distributor, I found that one particular pattern called Aquatex with its underside coated with Krylon Camouflage Green provided the look that I was seeking. I wrote a magazine article about this in 2008 and thoroughly covered the creation of the harbor in my book. It's wonderful to see this technique now being embraced by other modelers.