This thread got me curious, so I just did some micro-research on "low-e" glass.
It seems there are different kinds of energy efficient glass for different climates, and unfortunately the marketing people who advertise the stuff didn't bother to learn what "low-e" means before turning "low-e" into a buz word for "energy efficient". The smart thing to do would be to label these types of glass as either "low solor loss" or "low solar gain" glass.
It seems to boil down to two different transmissivity percentages, one for visible light, and another for infrared light (heat). When the sun shines in the window, most of the heat it brings is from absorption of visible spectrum light. This heats up the room, and the heat then tries to radiate back out through the windows as IR.
Energy efficient glass for northern climes was first to show up on the market, and its "low-e" is in the sense that it has "normal" transmissivity to visible light, but low transmissivity to IR light. The sun shines in and warms up the room, and the glass then blocks a decent amount of the IR light that would otherwise leak out through the windows. This is high solar gain, low solar loss glass.
For southern climates where energy efficiency means blocking the solar gain, there is "low e glass" that blocks significantly more of the visible light than normal glass does. This reduces the amount of solar gain through the windows. It also tends to have a low IR transmissivity, thus blocking the IR that is bouncing off of the ground and other surfaces outside the house. This glass looks slightly gray to the naked eye.
So, it would seem that for a solar water heating project, the best glass to have would be "northern climate low-e glass". Next in line would be normal glass, and worst for this project would be "southern climate low-e glass", as it is specifically designed to block the stuff you're trying to soak up.
jp