I am sure that everyone, but everyone knows about this, but I will post it anyway.
Watch what you drink when you are in India. Typically, we ask restaurants if they use RO water or filtered water. RO stands for Reverse Osmosis, and more and more households use these machines these days. There are grades of machines, and the tough ones claim that they can clean water that has been delivered by tanker. We do get water delivered through the pipe by the municipal department, and where this falls short, private companies make up the shortfall by supplying tanker water. This is poorly regulated, and so you need good machines to clean this water. There is a nice little nexus between local municipal officials and these operators.
So, first off, ask if the water in the restaurant is RO water or not. If the answer is in the affirmative, then we will not order bottled water. If the answer is in the negative, or we are not convinced (gut instinct!) that the machine is serviced, we order bottled water.
Don't just go for any brand. Be critical about which brand you choose. Bisleri, Baileys (less these days), Aquafina (from Pepsi), Kinley (by Coca Cola), Qua, Himalaya (Tatas), Catch are some good brands that you can trust. I did by "Q" water by the Sahara group when I was on a road trip last week. This is okay.
Check the seal!! Very important!
After you are done, crush the bottle. Throw away the cap and the bottle separately. I puncture the bottle.
I don't want some shady character refilling the bottle with lousy water, and selling it as genuine stuff.
I've suffered enough from water borne diseases, and this is the one thing you need to watch out for.