The Romans use to sing:
Ma che ce frega, ma che ce importa,
se l’oste ar vino c’ha messo l’acqua,
e noi je dimo, e noi je famo,
c’hai messo l’acqua, e nun te pagamo

My free translation:
Who cares, who minds
if the innkeeper put water in the wine
and we tell him, and we are like
you put in water, so we are not going to pay you

For the modern wine consumer, adding water to wine is maybe the quintessential form of fraud: we may forgive chaptalization, (de)acidification, sulfur dioxide, süssreserve and even questionable oak alternatives. All these practices are commonly acknowledged and described in wine courses and books.

The other day I happened to try an East Bench Zinfandel from Californian producer Ridge Winery, not really a cheap wine, as it fetches around 5,000 yen. Ridge Winery is famous for its detailed labels, where they list additions made during the winemaking process. Here it is.

Ok hand-harvest grapes; indigenous yeasts yes; naturally occurring lactic bacteria; tartaric acid all right; 1.2% water addition; oak and – Wait. 1.2% water addition? What is the meaning of this?

Well, welcome to the world of the common winemaking processes that everyone knows, but (almost) no one talks about in public: the addition of water to the must (mind, not to the final wine).

Legislation differs according to the Country, but as it happens New World laws are much more permissive about what you can do to your grapes in the winery. So while in Europe addition of water is forbidden at any stage “except where required on account of a specific technical necessity” (COUNCIL REGULATION (EC) No 479/2008 Annex VI, A, but the prohibition dates back to a previous 1999 regulation), according to the US Code of Federal Regulation, water may be used to actually reduce the acid level of the must, thus effectively changing the flavor profile of the wine (27 CFR § 24.178, under the euphemistic “Amelioration” heading).
California laws are a bit more restrictive than the federal ones: the State Code of Regulations states that water may only be used to facilitate normal fermentation, since the hot climate over there can result in very sugary must that yeasts may struggle to work (17 CCR § 17010). Well, after all in such a hot climate there is not much need to reduce the must acid level.

Anyway this is the meaning of the label above, as explained by Ridge Winery website:

Water: When temperatures during a zinfandel harvest rise significantly, this varietal can overripen quickly before there is time to pick all the blocks. If that occurs we make a small addition of water to those fermentors to rehydrate grapes that lost water to the vine in protecting it from the excessive heat.

Australia has a similar approach in its Food Standards Code (Standard 4.5.1), with the role of water limited to sugar dilution and thus to help fermentation.

Rainwater, potable water, water from boilers or treated river water are all possible candidates to the process, but a winemaker may want to distill before use, to remove chlorine/chloramines that may impede fermentation and stain the flavor of the wine. Water addition will also dilute your acids, so you may also want to acidify it if you are only trying to bring down the sugar density of the must. Flavors as well will be diluted, though if your grapes are so concentrated to need rehydration that could be a minor problem.
The common timing for this process is at the crushing stage, under the principle that adjustments to the must are better done sooner than later, leaving more time to the components to better integrate. A document by Brehm Vineyards points out that the exact quantity of water must be based on the volume of the finished wine, not the volume of the must that you are crushing. Precise calculations to determine this figure follows, but home winemakers may also take advantage of a convenient tool by Wine Business where you have just to input the volume of your must plus the starting and the desired final Brix. Et voilà!

In conclusion, I leave any judgements on this practice to my readers.
On my part I am mildly surprised that water addition is not addressed by the WSET or other programs: my Unit 2 Diploma manual from 2015 does not cite the issue (though this may change with newer materials), nor does “Understanding Wine Technology”, the book from David Bird recommended to Master of Wine and WSET Diploma students. The Oxford Companion of Wine has just a very brief mention. Producers of course will hardly tell you, as they hardy tell you when they use oak chips or staves. (1)

There is a lot happening in the winery and the idyllic image of vintners happily crushing their must and waiting for the action of nature to make their wine is naive (or obsolete at best), I see no reason to hide water addition, at least in education (for producers, well, I understand).
Thumbs up to Ridge Winery for not shunning the issue and letting the consumers know what happens during winemaking.

(1) A famous chilean winery gave me a good laugh when in the technical sheet for its Chardonnay stated that ageing had only been in stainless steel, to add in the tasting note: “aromas of wood”.