Diving Express

Diving Express is the big daddy of dive operators in Hong Kong, and by this I mean they seem to have their hooks in most of the consumer market here. They are indeed big, I believe they own three boats, one overnight boat out two day boats. They also have a shop in TST, but judging by the setup of the shop, you would have thought they are a mom and pop store.
Diving express is definitely known for their local dive tours and overseas by travel. Their local dive tours depart from Sai Kung, and every Sunday you can see the volume of customer throughput they have, simply by looking at the number of divers they jam onto each boat.
There are many other dive operators in Hong Kong, but you may be surprised to find that most of them book their boat diving on the Diving Express boats. If you are interested in diving in Hong Kong, you are better off making a beeline directly to diving express, as you will invariably end up on one of their boats anyway.
They also have a well stocked shop in TST. Like their boats, it is a bit of a cramped space with piles of diving equipment stack up on top of each other, but they do hold many products and most of the brands. The staff are knowledgeable (ask for Denny), most being divers themselves, and are willing to give advice as required. They also have a reasonable selection of diving camera accessories, which can sometimes be hard to find in Hong Kong. The dive kit is reasonably priced, and what they don’t have can be ordered.
As with many of these specialist dive operators, whether it be retail or travel, you are better off doing some research beforehand, or going with a buddy. Diving express unfortunately suffers from the sausage factory approach, which means as a novice diver you may be somewhat intimidated by crowded boats and shop.
Take care, and dive safe.