Everyone I know who went to India developed a sudden urge to use the toilet very frequently.
I spent a week in Sri Lanka followed by 2 weeks in India, not even the slightest stomach upset.
Probably got lucky but we didn't eat anywhere that wasn't busy and mostly avoided chicken. Ate street food, in small cafe type places, food on the buses and trains and a lot of curry.
Although KFC popcorn chicken is delicious!
Depends how far south you want to go and how much you're prepared to travel.
Fort Kochi is lovely, take a boat around the back waters and relax. Not much there so a couple of days is enough.
Punducherry is also nice, strong frence influence and a bit more beachy if thats what you're after.
If you just want to sit on a beach then Goa is probably what you're after although we avoided that so can't tell you anything about it.
If you're travelling be prepared for a country where no one knows anything and the trains and buses take forever to get anywhere.