No insta-drowning by create water
"Container" is not a technical term in D&D 5e, or at least I could not find any suitable definition in PHB; only examples: capacities of some containers, opening contains with mage hand, specific use in magic jar, etc. Thus, it has the commonsense meaning of the word. Going with a dictionary, we get
a hollow object, such as a box or a bottle, that can be used for holding something, especially to carry or store it
I would not place mouth in this category of things, though maybe native language users would. (And, just to state the obvious, the "object" in the dictionary definition is not the "object" in the specific technical sense of D&D 5.)
However, there is nothing stopping a fantasy creature from being clearly a container: Animated objects and mimics are the most obvious candidates, but certainly an inventive fantasist can come up with more such creatures.
You need a clear path to the target. Unless the person is just standing there mouth open, this is not quite obvious. Furthermore, the container needs to be open, which at least means the mouth needs to be open.
How to allow the idea anyway
What happens if you get your mouth full of water? On a good day (with a successful save), you spit it out. The bad case is that you happen to try to breathe some of it in before you body figures out what is happening, and you'll get nasty uncontrolled coughs. Probably a disadvantage on most stuff for one turn. A nasty blunder in a high-stakes social situation, anyway.
Target who is not keeping its mouth open most of the time might get advantage on the save to represent that the spell might fail due to the "container" not being open - asking for a separate attack roll is also possible, but more cumbersome, and probably only worth it if even a successful saving throw has a definite effect on events, such as if trying to impress someone in court.