As a person who has dived the region before, just forget about diving in Langkawi, its a waste of time. Anything in the west coast channel of the Malacca straits is just muck heaven and not the good kind like Lembeh, Indonesia for macro.

If you are willing to slightly "downgrade" head to Semporna and then go on a 45 min speed boat trip out to Mabul islands, you have the choice of Kapalai Dive Resort (i dont recall seeing a beach there), or on Mabul island itself, Mabul Water Bungalows and Sipadan Water Village, both have over water rooms and i recall the beach is alright.

August, its still alright for the east coast of Peninsula Malaysia. Usually we call it quits mid-September for the diving season there. I would say check out the Taaras (https://www.thetaaras.com/), private beach on the island of Redang, sand there is one the best in the region and diving is decent enough.

Also echo what Nov11 said for Langkawi, overall the Datai is great and also check out Four Seasons as well. That side of the island has better beaches.