About the Visa

No, you don't. Your Chinese visa, that you have obtained these days will perfectly cover your all trips within China, including your trip to Sanya. I understand that people are sometimes a bit confused when they plan their travel to Hianan island, but they should not be because Hainan province (island) is considered to be a part of mainland China and consequently your trip to Sanya is the same as it would be a trip from Beijing to Shanghai or Xi'an. Purely domestic and beside your visa, you don't need anything else. Also, your visa will not expire when you travel to Hainan. You can return back to the continent and your visa will be still valid. To conclude, just treat your planned visit to Sanya as you would be considering a trip to some other more conventional Chinese destination.

Thank you so much for the response ! :) And last thing , do I have to submit a hotel bookings when I land to sanya ? As I am staying in a friends house :) Thanks ;)

No, you don't need to do that, but since you will stay at your friend's house, you will have to register at the police station (Public Security Bureau) within 24 hours from your arrival in Sanya. Please, don't forget to do that and your friend should definitely assist you withe the registration since it is almost impossible for a non Chinese speaker to do it by herself/himself.