I used to live in Langkawi for three years and there is very little diving in the area.
Where are you staying on the Island. It does not make much difference as the local dive shops will pick you up.
The dive shop I worked for was Pro Dive. They are based in Pantai Cenang (Chenang Beach) Very good operation.
They are the only dive shop to go to both dive sites.
Pulau Payar is the main dive site, and it is very good. But there will be hundreds of snorkellers and thousands of Japanese. This does not really affect you as you will be underwater and they will be making froth on the surface
after the first dive you will be taken to the beach and there you can feed the baby black tip sharks and snorkle. Maybe see George the HUGE iguana (6 foot)
Then the second dive (dependant on weather) will be either around the wrecks or under the platform. Go to the wrecks if you can ... viz is about 10-12 metres but you can see adult black-tip sharks 9 times out of ten.
The other, much better dive site is Segantang. But they can only go there if
a) There are only divers on the boat (there is no where to land, so you are on the boat
b) the weather is calm
c) they get port permission to go.
It is phenomenal! Viz is better, the marine life is pretty much undisturbed and we have seen a pod of 40 pilot whales there.
On the way home they will go hunting for pink dolphins so that you can watch them. Cant get anywhere near them as they are too nervous but the leader of the pod is bright pink, you can see them a long way away
Also you may get to see Tasik Dayang Bunting (The lake of the pregnant maiden) It is a fresh water lake inside an island in the middle of the sea. Lots of fun.
Then a slow ride home to enjoy the sunset.
The owner's name is Andrew Lee (PADI Course Director), and the boat skipper is Sharreef (nutty as a fruit cake) both good guys.
If you fancy a bit of scary bed time reading ... then check this out.
http://www.dsproffitt.pwp.blueyonder.co.uk/home/Stories/Bubba home.htm
Generally Langkawi is an idyllic island with great people and superb food. If you want to PM me with where you are staying then I can give you a list of restaurants to eat in, things to do, and people to say hello to.