Here's a dirty little secret about diving in Thailand: near-shore diving in Phuket is not the best you'll find. The best sites are an hour or more by boat, water visibility can't compare to the Caribbean, the reefs are pretty degraded and local fishermen have taken just about everything bigger than a foot or two. All that said, you're diving in Thailand. You'll see lots of moray eels, probably some sea snakes and rays, and maybe a leopard shark. It's awesome. And Dive Asia endeavors to make the above-water experience nearly match the underwater one.
We went on four trips with them over 10 days and never had a bad experience. The daytrip fee (3400 to 3900 THB as of this writing) compares pretty well to other local operators. Included is transportation to and from your hotel. Also included is lunch and dinner (for three-dive trips), meals which deserve a review of their own: the food was some of the best I ate in all of Phuket. The dive boat is a cattle barge -- you'll be sharing space with 20, 30 or more divers, on the most crowded days -- but my observation is that all the local operators work that way, and most of the other boats we saw did not look as comfortable or capable. Even with many divers and snorkelers aboard, there's plenty of room to spread out on the boat's two decks. The dives themselves are run with military-like efficiency and precision, mitigating any problems crowding might cause.
Dive Asia's guides are great. From what I could tell, most speak at least passable English in addition to other languages (the operator is especially German-friendly) and they're all excellent spotters. Most of them also appeared to double as instructors. My wife completed her certification during our first two days of diving, and I have to single out her instructor, Declef, who couldn't have been more patient and friendly. I hope I didn't misspell his name.
In short: they're the biggest and most visible dive operator in the Kata area for a reason. Use them.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.