Unawatuna has beautiful water and good for swimming. The beach can be narrow at times, if the water is rough but December should be nice...And Unawatuna gets crowded too.. I have stayed at the Banana Garden, at the east end of the beach..It's less crowded there..The room rates range from $18 onwards....
I went to Tangalle last month to check out the guesthouses for my trip next week. Palm Paradise cabana rooms are very nice but I didn't think there was enough beach. On the other hand, the beach near Ganesh Gardens stretches for miles east.. We stayed at the Lagoon Paradise Beach which is next to Ganesh Gardens and behind the lagoon. It was a nice simple place, but if you want to be right on the beach, Ganesh Gardens should be good. They have hammocks in some of the rooms and also on the beach. If it wasn't for the Lagoon, I'd prolly stay there!
As celticmuseum said, Tangalle isn't very good for swimming. We went to wade around this little pool like place formed behind some rocks, down the Medaketiya Beach. The beach is very beautiful there although can get very hot since there's hardly any shade. Loved watching the sunrise and the fishermen!
If you want any more info on Unawatuna or Tangalle, you could always PM me :) We are going to Tangalle around the 22nd Dec.
cheers