There are lot of beaches in South India to explore. Just sharing my personal opinion here. May vary from person to person.
1. Auroville Beach, Pondicherry
2. Varkala Beach
3. Gokarna Beach
4. Paradise Beach, Pondicherry
5. Marina Beach, Chennai
6. Ramakrishna Beach, Visakhapatnam
7. Kappad Beach, Kerala
8. Om Beach, Gokarna
9. Dhanushkodi Beach, Rameshwaram
10. Marari Beach, Alappuzha
11. Bangaram Beach, Lakshadweep
12. Havelock Beach, Andaman
are some among them.