In the past it was a strategic point because it is the only water source of all the area, so boats where often stopping there, and pirates where attacking them meanwhile. It's why there is an old fort on the beach, to protect the boats.
San Pedro is not accessible by car, only by hiking 2 hours from Las Negras, or by boat. If you don't want to walk you can pay the taxi boat to go there from Las Negras, but it's quite expensive. Or you can be lucky and hitch a boat of locals coming to buy stock at the village, but don't count on it.
On the beach itself, there are around 25 houses built by people living on the beach. It's a very hippyish chillout place where clothes are optional. Only a small amount of people are living there all year. There are some houses that are actually bars, where you can get a beer, or some food. You can usually meet really nice people there, and the atmosphere is great.
Camping on the beach is tolerated, and you shouldn't have any problem doing it. There is a fountain with fresh water from the mountain, where you can get drinkable water and wash yourself. The beach during July and August can be crowed and quite dirty.