I am under the impression that the word staff is uncountable/singular when referring to a collection of employees in a company. This is corroborated by some online sources I have found: Macmillan and this article.
However, I just heard this in a YouTube video (at 1:36):
These non-litigious companies typically have much smaller legal staffs.
It seems the person in the video uses the plural form to talk about departments at different companies, thus using staff in a way similar to I'd like two waters, please. But is this usage common?
I clarified which usage of water I was talking about. As one of the answerers CJ Dennis said in a comment, "It's the classification of individual words that drives grammar." I very much agree with this opinion and wonder if the connection I loosely made in my line of thinking is part of the classification at issue. Of course answers don't have to address the pluralization of water.