I think you would be looinsing your time in Langkawi. I stayed in Langkawi quite some time ago and went looking for diving. I found there are two options :
Pulau Payar is done on day trips (2hours speed boat ride south) plenty of blacktip sharks and tourist attractions, IMO not worth the trip.
On the other hand Koh Lipe is facing Langkawi on the thai side (also accessible from Pak Bara and Satun), the beaches are probably some of the best in Thailand, the diving is ok, but not flabbergasting. it is not possible on day trips, you must better stay in Lipe.
On the South China sea side, Perhentians are really gorgeous islands, the diving is fine though not "worldclass", though there are interesting sites like Tokong Laut or Tiger Rock which are very pleasnt, lots of coral fish, nice soft corals. No biggies though.
Redang is near to the Perhantians, a little more upmarket, never dived but siad to be one step more intersting than Perhantians. Nearer to on the east coast, Pulau Tenggol is supposed to be hands down the best site of continental malaysia, unfortunately AFAIK it is only catered from Tanjong Jara resort which easily makes a hole in your wallet.
Continuing down south is Tioman, very popular with singapore weekenders, nice beaches but quite tame diving though I have read in the latest AsianDiver magazine some new macro spots have been discovered lately... if you are a photographer and a macro enthusiast this might be for you.
Borneo/sabah : to be honest, there are few places in the world that can compare Sipadan dived on a good day. Walls of Jacks and Cudas, loads of sharks and turtles :shocked2: plus any small animal you name it, the guide will make himself an honor to find it on Mabul or Kapalai. :cool2:
Borneo Divers is for me unbeaten of price/quality ratio (been with them 3 times), given that everything is included, and the dives unlimited it is almost given for a die hard diver.. maybe more expensive for a family but still even a kid can snorkel on Sipadan!
Some photos of my latest Sipadan dive trip end of february :