I have lived in India, so some of my first hand observations whilst I lived there.
I have been ill a number of times from those 5* hotel (and yes the Taj). Try and keep away from the hotel buffets, get items cooked fresh, inc breakfast.
Water I had a RO tap in my kitchen but you can still get ill, I relied on drinking bottled water only and RO was for cooking. Drink only bottled water and check the seal yourself, no ice for me or ice cream. (unless I made it)
Curd/paneer, I made my own, but generally its fine to eat.
Eat foods that you can peel/boil/cooked fresh, but watch those spices if your not used to it, lime juice with sparkling or still bottled water is a good thirst quencher, but ask for it without sugar.
Water in showers, now this has been talked about in many travel blogs, but my friends and work colleagues always told me to shower with my mouth firmly closed and be careful if you wear contact lenses to keep eye closed due to water born parasites (obviously not wearing lenses in the shower), some even went as far to advise mouthwash when in the shower, never did go that far.
Swimming pools even in those high class 5* ones attract men to just sit or stand at the side and stare at women, its a bloody pain the the backside, and I have many a time to the pool guys to get these men moved, and had a hissy fit when one guy decided to sit on the edge on my sunbed on my towel......dozens empty all around.
You will be asked for selfies, if people are being polite, or just take your pic anyway and sidle up to you and the lack of personal space can get very uncomfortable with people getting too close.
Clothing, I wore a lot of loose linen trousers (never wore dresses or skirts) and kurtas, a long tunic kind of top which you can buy a lot of when your there if needs be, cotton or linen, never sleeveless and more covered up up the front. You can buy dupattas which are scarfs to cover up more if needed. I always had cotton slip on socks to wear when going into temples etc. your feet will either be very hot or get very dirty. Birkenstocks are always my choice of footwear, but general flip flops by the pool or beach. But a lightweight walking sandal would be fine. Most of the bigger supermarkets sell clothing as well as general clothing shops, but I do like Fabindia and Anukhi.
Toiletries, now of course you can get all the toiletries you need there, but usually not the brands that are important, good quality sun lotion face/body/head if you dont wear a hat (like me), deet deet and more deet, Joungle Formular is the way to go, the local Osmos was usless for us and at first got bitten badly. Also aftersun, I am particular about my brand and dont like any other. Make sure you cover up at dusk, and mozzies have a preference for darker coloured clothing.
Make sure you have tissues in your bag as many places have either a squat toilet and dont have paper.
You can get anti diarrhea tabs from any pharmacy and they work well, but take some paracetamol/ibuprofen, wet wipes, hand sanitiser, some plaster inc ones for blisters. And of course any of your regular medication if you take any as well as a copy of your prescription.
Keep away from the dogs......
Thats all I can think at the moment....