A sea of 500 tents covered the white sands of remote Ham Tin Wan in Sai Kung during a recent holiday, turning one of Hong Kong’s most scenic beaches into a makeshift campsite for crowds of overnight visitors. Some of the campers had joined tours from mainland China for the Labour Day “golden week” holiday, sleeping in rows of identical tents and gathering around camping tables for hotpot dinners, with ingredients hauled across the border by their guides. By morning, the sink in the beach’s only...