The whole waterfall sidebar really illuminates people's approaches to the game IMO.
RC is basically standing by the idea that the DM did nothing wrong and the module is poorly designed. In very limited circumstances, you can see through a waterfall, therefore, you should be able to see through it in this circumstance thus the module designers were out to lunch.
I take a totally different approach. I always look at the DM first. Looking at so many rules discussions, "user error" is probably the number one suspect for problems.
Not sure what this has to do with the topic, but, I think it does nicely illustrate why so many conversations here grind to a halt. We're just coming from polar opposite assumptions. I always assume the DM is wrong first. Because, quite honestly IME, it is the DM who misread, misunderstood or was just plain wrong far more than the mechanics or modules.
I think it boils down to whether or not you've had the joy of playing with really, really bad DM's in the past.
To me, the idea that you can see through a waterfall, on either side, is pretty remote. Unless, as cwhs01 points out, the water is falling perfectly straight, there will be lots of white water and you cannot see through it, and likely you won't hear through it either. Since the module advises that those behind the waterfall won't come out unless specifically contacted, which do you think is more likely? The water is falling perfectly and allows people to see through it, or the water is a typical waterfall, loud and opaque?
To me, I'd go with the latter because, given the context of the adventure, it makes the most sense.