Hi, scuba diving in both the places Phuket and Ko Phi Phi is a beautiful exprience. But dive sites around Phuket that are very suitable for training.
The dive sites close to Ko Phi Phi and the Similans, it is suitable for more experienced divers only.
I went Scuba diving when Maya Bay was closed. It's still really great. They did take us close to Maya Bay for the shallow water training and first dive. The second dive was near Bida Nok. Scuba diving in Phi Phi islands was considerably cheaper compared to the one in Phuket and I did read that underwater clearness is better in Phi Phi.