Just written something on beaches in HK. Here's my pick. Whaddya reckon?
(Searching net a bit, I found positive comments re Turtle Cove; haven't been there. I kept to beaches with lifeguards.)
I like Cheung Chau: the two beaches on the east coast are good. Tung Wan is the larger, and more popular; smaller Kwun Yam Wan (Afternoon Bay), just south of the Windsurfing Centre, is generally a little quieter and is especially good for a swim late on a fine afternoon.
Kwun Yam boasts a small hut that serves drinks, with a smaller crowd of regulars on many summer afternoons (sometimes, I'm amongst them). The hut also offers kayak rentals; but when my wife and I rented one, it sank thanks to a couple of holes that had been crudely patched with duct tape (!) - I've since rented from the Windsurfing Centre, without mishap.
You can reach Cheung Chau by First Ferry, chiefly from the Outlying Islands piers in Central. To find the beaches: walk ahead and slightly right from the ferry pier, then follow the small road across the island (narrow here), and you'll arrive at Tung Wan. Turn right here if you want to try Kwun Yam Wan.
Cheung Sha on southern Lantau boasts two long, beautiful beaches. Looks exposed to southerly winds, so perhaps best avoided during southwest monsoons. There are open air restaurants, including the Stoep, just above the tideline of the eastern beach.
Buses from Mui* Wo to places including Tai O pass Cheung Sha; there are stops right above the beach.
Repulse Bay on southwest Hong Kong Island has one of the most popular beaches in Hong Kong. It isn't splendidly scenic, thanks to all the high-rises on the hillsides, and the sand is a tad coarse, but is easily reached and has plenty of facilities. At the southern end is a big statue of Kwun Yam, Goddess of Mercy, as well as a longevity bridge (cross to supposedly extend your life. Hmm).
Nearby Deepwater Bay is generally rather less crowded, but with a road close by it's no tranquil haven.
Buses from places including Central run right by both these beaches.*
Shek O, on Hong Kong Island's east coast, is a village with a great looking beach to its south. This beach looks wonderful when there's some surf rolling in. I've seen it packed during summer weekends, but as it's relatively far from the city and few people live here it should be fairly quiet on weekdays.
A little to the north is Big Wave Bay, which is in a more rural setting, and is Hong Kong's only designated surfing beach. (The "big" waves would disgrace Hawaii or other real places for surf dudes; surf's only really up during strong winds, such as with tropical storms or typhoons.)*
There are buses (9) to Shek O from Shau Kei Wan (MTR station), also a public light bus service that stops at Big Wave Bay; and on Sundays and public holidays there's a bus service (309) from Central to Shek O.
The two Clearwater Bay beaches are in fine settings, on the east coast of the Clearwater Bay peninsula (east of Kowloon). I've only tried once for swimming - and was surprised how murky the water was; maybe I was unlucky, as both ranked Good last year.
Bus 91 from Diamond Hill MTR station serves both these beaches.