There are over 70 temples in Hong Kong dedicated to Tin Hua, the goddess of the sea. The oldest at Joss House Bay was built in 1266 and is considered the most sacred while the one in Causeway Bay is known for it's architecture. If you're only visiting one though, the temple in Yau Ma Tei is probably the most popular and accessible.
Yau Ma Tei was originally built on the harbor before land reclamation drove the shoreline far away. Now it sits in the heart of the popular Temple Street Night Market. It's known just as much for it's beauty, history and Wall of Nine Dragons as it is for being the social heart of the neighborhood. In front of the temple you'll find a row of willing fortune tellers, inside Chinese chess players congregate in the square.
Oddly, this temple is one of the most popular filming locations for Hong Kong gangster movies.