My time in the southern part of India was short and sweet but it definitely has a lot to offer. I was lucky enough to spend the day on the backwaters of Kerala near Alleppey. This is the main spot to rent a houseboat and spend the day on the water.
If you are looking for beaches I would highly recommend the beaches of Goa. Or to find out more about the area, check out this guide to Kerala.
Mine was a simple Indian house boat as I only spent the day on there. A traditional southern indian lunch was provided as well. It was really delicious. The boat ride was great but let me tell you when it stopped or slowed down it was really too hot for me. You could also rent a villa in the area to stay at while in Kerala.
Now if you want to visit the north of India check out this post on Sikkim.