Yes i agree that August is not the best time to dive Phuket, but if you made up your mind so i would recommend to dive with
Marina divers
I did dive with them last Jan and they have a variaty of packages and good prices, the service, the staff, the eat and the boat was pretty nice and i would recommend anyone to dive with them, although you can still search for more Dive centers to dive with but you wont get a higher quality as they are giving a good one in this center.
I hope you will choose the best for you, enjoy it.