1. Ordering, Asking, or Serving Water
In a restaurant or bar, the elliptical construction "a water" is acceptable, and understood to mean "a glass of water". One can often hear a request like, "Two waters and a coke, please!" or "Two cokes and a water, please!":
Examples of ordering water:
- "May I have a water?" = "May I have a glass/cup of water?"
- "Could we have two waters?" = "Could we have two glasses/cups of water?"
Some other examples from corpus:
- "I would like a beer. What do you have on tap?"
- "...And one for my friend." (Where one references "a beer".)
- "... I can buy a juice or a water or a soda or a milkshake or a cappuccino" (The impossibility of a behavioural welfare economics, Comment by username "ianlee", September 25, 2012 at 01:40 PM)
2. Extended Discussion on Asking for Water
This can extend to other situations where a glass/cup would be available and a sense of serving is understood. In a home during a party in which food and drinks are served, or especially during a cocktail party, offering or asking for "a water" would be common. However, if you were just a visitor/friend, it's common to ask for "some water", especially if you don't mind it coming from the tap. Otherwise, one would ask for a bottle of water.
Situations in which "a water" would probably not be used:
- The man went to the well to get me some water. (Focusing on a massive source shifts the meaning to the uncountable noun form of water.)
- He poured some water into a glass. (Focusing on the water itself being poured refers to the water itself.)
- He poured a glass of water into another glass. (Focus is on the water being poured again, so "a glass of" is required.)
- Set your smart phone to remind you to drink [some] water every hour. (One would not say "drink a water every hour".
3. Other uses of "a water".
"The classification of a water as potable (i.e. fit for drinking) or otherwise is based on the requirements of the European Communities (Drinking Water) (No 2) Regulations 2007 (S.I. No. 278 0f 2007)...Some coliform organisms are able to grow in soil and are not of faecal origin thus a second analysis is carried out for the presence of total coliforms, giving an indication of the general level of microbiological contamination of a water....Therefore, the more vegetable matter there is in water the greater the colour. Exceptionally, natural colour may arise from the presence of colloidal iron/manganese in a water but organic matter is almost always the cause."* (Water Services - North Tipperary County Council
Comment: This refers to a body of water as a whole, rather than a portion of water.
"An image of a water lapping a flood-level indicator on a country road may be used in a news story." (Don’t let your CMS wreck your content, part 2)
Comment: This similar to referring to "a body of water". In this case, "a water" is being used to refer to the iconic picture, stopped in time. The "body-of-water" as a whole, is lapping against something.
Other uses of "a water" in which water is used as a modifier or compound noun:
- A water well.
- A water filter.
- A water soluble vitamin.
- A water and acetone combination (Note that "combination" is the primary noun.)