I once had to explain to a man I'm "friends" with (I use that word loosely in this context as I don't like him as much as he likes me but that's a whole other issue...) that a medication isn't a "water pill" just because you take it with water, it's a water pill because it flushes water out of your system. I'm absolutely floored at how many things this guy doesn't seem to know about or understand. This is one of my favorite incidents with him because I thought everybody knew that. Obviously not.
I'm an adult, and this is the first time I've ever heard
of a "water pill." But then again, I've never needed medication to flush water out of my system, either, so it's not really relevant to me. I expect adults to learn about the medications that they themselves take or that people under their care take, but otherwise I don't see why it would be common knowledge.
Incidentally, if I'd had to guess based on the name, I would have assumed it was either a pill that contained/appeared to contain water (something along the lines of a gel cap) or one designed to increase
the amount of water in the body. Calling a diuretic a "water pill" seems quite illogical to me.