"The Lords of Dogtown" originally practised their tricks (a.) on the beach.
The sentence that indicates that the kids practised their tricks on the beach before skateboarding in an open drainage canal and empty pools is "The beach was ruled by surfers, but in the afternoon, when the waves died down, some of the surfers, or their younger brothers, fooled around on skateboards". The movie "The Lords of Dogtown" is based on the true story of of a group of skateboarders from Venice Beach, California in the 1970s.