There is no true "dive resort" in Phuket. Yes, hotels like the Meridien/Kata Thani/Marina resort/Club Med have dive centers inside, BUT I guess for a true dive resort you want to have the dive boats right there at the resort too. The boats from dive centers that are in these hotels all depart from Chalong Bay, and are not only for resort guests - we book people on the Euro Divers boat and Sea Fun Divers boat. Of those mentioned my vote would be for Sea Fun at either the Kata Thani or Le Meridien. I guess Sea Bees in Chalong is another option, especially if you speak German.
The only 2 resorts in the area that I would call "dive resorts" - ie, have a dive center in the resort and the boats depart from the resort and are for resort guests only. There is a dive center at The Racha resort based on Racha Yai island, and there is a dive center at the Phi Phi Island Village. These run trips for their guests only. The Phi Phi Island Village dive center is run by Joerg and Yvonne who have been in the area for many years and are very nice people.