Phuket has PLENTY of dive operators there. The prices are quite competitive there and prices are normally listed so not likely to kena chop like carrot.
So yah, u can just walk in there. They have this jetty place like Mersing where all the operators gather. Like in Sg, some of the operators group together to save costs. So sometimes you find that it doesn't matter so much which particular op you book with. you still end up on the same mini bus and go on the same boat. hahaha
Well, there are several dive sites near phuket itself. There are various pho this and pho that islands. Shark Point is a fav for ppl who wanna see sharks. Last time I went there, all the other pairs of divers got to see shark except me Next trip to phuket try again bah.