Ap Lei Chau Market is located on the Ap Lei Chau Island, which is right across from the Aberdeen Wholesale Fish Market. The wet market has some of the freshest and greatest selection of seafood in Hong Kong available to retail customers (the Wholesale Market isn’t open to public, unless you have connections). In my last visit to Hong Kong, I went there a couple of times to get live seafood. Upstairs to the market in the cooked food centre, there are also cooked food stalls, like 珠記, 栢記, 耀記, etc who can prepare and cook your just-bought seafood to your specifications, at very wallet friendly prices.
The available seafood is dependent on the catch, but selection is generally wider than most wet markets in Hong Kong. Some of the more expensive fishes more commonly found in high end Cantonese restaurants, like Australian Coral Trout, Lunar-tailed Coral Grouper 燕尾星, can be bought live here. Everything live is held in trays and buckets with water continuously circulating via tubes. There are also frozen seafood. Just walk around the market, and buy what looks liveliest and delicious that day.
Reservations for tables at the cooked food centre are recommended on busier nights during the week.
Easily accessible from the Lei Tung MTR station exits A1/ A2. Or take a taxi ride after a day-excursion from nearby Ocean Park. Or better yet, take a sampan ride from along the Aberdeen Promenade to cross the harbor from Aberdeen to Ap Lei Chau, which lands you right outside the market.
My ~2 lb coral trout, just below HK$300, which would have cost me an arm and a leg back home in the US: