One of Hong Kong's hidden treasures; I must confess it came as a bit of surprise when I first visited them. They are clean well maintained and have Life Guards on duty until six in the evening during the season, April to the end of October. All beaches have changing rooms, showers and a barbeque area. Park n' Shop and Welcome supermarkets sell all the necessary items you need.
The majority are located on Hong Kong and the outlying islands; there are some good ones Kowloon side and well worth a visit.
The list isn't exhaustive but here are some I would check out.
Hong Kong, Repulse Bay, Deep Water Bay, Shek O, Stanley.
Lantau (island) Discovery Bay, Silvermere Beach (Mui Wo).
Cheung Chau (island) Beach, Lamma (island) Hung Shing Ye.
Kowloon, Sai Kung.