Given that it’s an island, Sri Lanka has lots of beaches, and we were eager to explore them. Both of us had an image in our heads of a long, deserted beach, with warm clear water. The beach in Colombo was unusable and smelled like sewage, so we waited until we got to the southern coast to swim in the Indian Ocean. The water was very warm and incredibly salty, which allowed us to float without having to tread water.
The first beach town we stayed in was Tangalle. The nearest beach to our guesthouse was nice, but small, and had a faint smell of sewage.
Next, we took a bus to Unawatuna. Some Aussies we had met said that it was a small town with a nice beach, which sounded great. Their description was accurate; it was a small town, and it had a great beach, but it was full of Russians. We definitely don’t have anything against Russians (we loved our trip to Russia), it was just not what we were expecting. The beach, crowded with loud discos at night, seemed more like Miami than remote Sri Lanka.
However, it was fun, and the beaches were beautiful. The town actually has two beaches. The more secluded one, called Jungle Beach, can only be reached by hiking up and down hilly paths.
There is a big reef just off the shore, which made it uncomfortable to stand in the water, but that wasn’t a problem, as the salty water allowed us to float easily. Some monkeys were jumping through the trees while we were there, which added character. The main beach, along the bay is long and sandy. 
As we were walking along it one night, we came upon a sea turtle laying eggs, and watched as she used her strong hind legs to dig a hole into which she laid ~100 eggs. She cried as she laid the eggs. Really. Sea turtles cry during egg laying. Or, they emit salty secretions that look like tears. Then, she used her flippers to cover up the eggs and propel herself back into the ocean.
The whole process was magical, and took about an hour. After she was back in the ocean, a bunch of locals raked over her tracks and asked us to make footprints in the sand in order to cover up the evidence of her presence, as they said people will dig up and eat the eggs. Which is really sad.
The final beach location was exactly what we were looking for. We rented a cabana in the jungle in the village of Ahangama. We biked to the beach, which was sandy and had clear turquoise water. 
The water was so shallow, we were able to walk out 100m. There were waves offshore, which some local guys were surfing.
Our attempts to try our hands (or feet rather?) at surfing, without taking lessons, didn’t fare so well, in the short hour that we had the surf board.
Nearby, stilt fishermen perched on sticks in the water, attempting to catch their fare.
Ahangama was a lovely, unspoilt beach, and was definitely our favorite spot.
Beer of choice: Lion (again)