I was advised by both locals and colleagues who'd travelled to India to even be careful
which bottled water you drink, as some (such as refilled glass bottles provided in hotel rooms) are filled from tap water.
The rule of thumb I was given was to stick to brewery brands , as most have related bottled water brands.
These have filtration and other sterilising processes as their beer counterparts.
I would think the 'drinking water' taps are there for locals who are used to the bacteria which is in the water.
An 'initiation ceremony' I've heard of, for people who have permanently relocated to India is to be handed a glass of tap water and told to spend the next week at home ill...
once you've been ill you're good to go, but if you're only there for a few weeks it really wouldn't be good to be out of action for a week.
travel.stackexchange.com/users/2000/dsample
No, tap water is usually not safe to drink in India.
Households commonly buy large, office cooler type bottles of drinking water or have in-house filtration systems.
So if it's a normal tap, then don't drink from it.
The exception is if the tap has a cooling or a filtration unit next to it.
(I can't find a free image for this to include here.)
These could be a small single unit with tubes leading to the tap, or for larger multi-tap units then the cooling compressor should be obvious to notice.
travel.stackexchange.com/users/108/Ankur Banerjee
I just spent 7 weeks in India and while I did not drink the normal tap water, I did drink the water from the Drinking Water stations in railway stations or near temples.
I also drunk the water served in restaurants.
I did assume that this water while not coming from bottles has been through some sort of filtering system.
I did this from day 1 and have had no health problems whatsoever.
Of course there is always a risk, but most people I spoke to who got sick in India did not drink the 'public water'.
travel.stackexchange.com/users/1317/Peter Hahndorf
Drinking tap water in India is not always safe, especially at railway stations.
However the tap water available at most of the monuments is safe to drink.
I advise you to drink bottled water in trains.
At many places there is an arrangement for filtered tap water by Indian government.
You can trust that water.
travel.stackexchange.com/users/2321/Jaguar
Drinking unfiltered water in India is the number one cause of illness for foreign (and local) travelers.
Consumption of tap water in India, unfortunately, carries the risk of water borne diseases such as Dysentery and Typhoid.
Even with bottled water, ensure that you purchase from a reputable outlet, and not from a street hawker, to avoid the risk of contamination.
On the other hand, if the water is heated to a boiling point (as in the preparation of tea/coffee), it will be potable.
PS>
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I haven't heard much about the SteriPEN, but even if you had it, I wouldn't trust implicitly.
travel.stackexchange.com/users/268/rs79
I'd advise not to drink tapped water from public places.
Water at railway stations/bus stations needs to be avoided.
Coming to museums and places with monuments, it depends on the hygienic conditions at that place.
Thus, bottled water is advisable.
In India we have places where you can get Rs.5000 worth shoes by Nike for Rs.500 by name nikee.
Duplicate brands or parts of almost everything, can be found in India.
Same applies to bottled water too.
Watch out for duplicates or local made's like 'Aqaufeena'(Aqaufina-->Trustworthy brand),'Besleri'(Bisleri--->Trustworthy brand).
travel.stackexchange.com/users/3113/Anuj Balan
If you are in Mumbai where the municipal corporation supplies drinking water to you then it is probably the safest water you will get anywhere in India.
If there are 2 different water supplies (one for sanitation and one for drinking and cooking)
then it pretty safe to drink the tap water.
travel.stackexchange.com/users/93798/Akshay
I ordered  SteriPEN  before I started my India trip but unfortunately it didn't arrive in time. I'm only drinking bottled water here but am interested whether tap water in train stations and some monuments is safe to drink or is the board which says  Drinking water  just put there as a decoration or is that water really safe.  Related:  Drinking tea and coffee on the street
Drinking tap water in India
travel.stackexchange.com
<health><india><food-and-drink><tap-water>
travel.stackexchange.com/questions/5991
travel.stackexchange.com/users/1593/rlab
Drinking tap water is not advised, even if some locals drink it and are used to the bacteria. Drinking unfiltered water in India is the number one cause of illness for foreign (and local) travelers, and households typically buy large cooler-style bottles or have their own filtration systems. Public sources like train stations are not necessarily safe, although Mumbai is probably the safest municipal source in India. Even when buying bottled water, make sure that itis from a reputable outlet to avoid the risk of contamination. It is not considered safe to drink tap water in India. Indeed, bottled water is always recommended and, most importantly, a reputable brand. The one exception may be Mumbai where the water supply is safe.
Tap water is generally not considered safe to drink in India. Except in Mumbai where the municipal corporation supplies the water.  Bottled water is recommended and care should be taken to buy a reputable brand.