The best sand beaches for swimming and water sports are at Unawatuna. There are two lovely spots to sit and sunbathe in relative privacy at Light House Beach down a handful of steps or by the main working Lighthouse in the Fort or Lady Sea Baths, where you will find local women swimming fully dressed due to the predominantly Muslim occupancy of the Fort.
A nice place to enjoy Galle beaches, offering breathtaking ocean views, is the many seaside hotels/motels/restaurants. Jetwing Lighthouse is within easy reach of the Galle coastline and its beaches where one can soak up under a beach umbrella the whole afrernoon. We had a lovely Sri Lankan curry lunch buffer there with a view to kill. http://www.jetwinghotels.com/jetwinglighthouse/galle-beaches.html.
Another place we visited was Wijaya Beach Restaurant. Simpy stunning. http://www.tripadvisor.com.sg/Restaurant_Review-g297896-d1818302-Reviews-Wijaya_Beach_Restaurant-Galle_Galle_District_Southern_Province.html.
http://kohomban.net/galle/beach-01.shtml