On Rottnest Island off Fremantle, The Basin is the island's most popular swimming spot - a clear, shallow lagoon of turquoise water ringed by limestone reef, close to the main settlement and reached on foot or by bike. The reef shelters the pool from the swell and teems with fish, making it ideal for snorkelling, and the white sand and calm water suit families and children. Quokkas, the island's famous small marsupials, are never far away. On a hot Perth day it is the classic Rottnest swim, an easy ferry ride from the mainland.