Not quite true... having trouble getting water in my current embark, and I'd really like to get something going for my hospital. It said on the embark screen that there was an aquifer, but there isn't... just two murky pools on the whole map. Making more would be worthwhile. But it's super rainy around here, so channeling and creating artificial murky pools would actually be kind of nice (even if I need water purification... not sure if I will or not).
So in general, if you have trouble getting water in a rainy area... it could help.
Do you have a single biome embark or a multi biome one? In the latter case you may have an aquifer only in one of the biomes.
The DFHack command "reveal" can be used to show if you actually have an aquifer and where it is (you should probably crash DF afterwards as reveal/unreveal are said to be capable of creating nasty side effects). The DFHack command "prospect all" shows the level(s) of any aquifer, if I remember correctly.
Normally at least one cavern contains a lake which can be used to siphon off water into a safe cistern.
I recently had a two biome embark with an aquifer in both of them, but somehow I managed to strike the earth where there wasn't one, and I still don't understand how that came to be. A fair bit of probing allowed me to find both of them eventually, though.