Baker Girl
I've certainly both stayed and dived in Krabi, Phuket and Phi Phi.
In Krabi most of the dive shops are on Ao Nang beach but some have "offices" on West Railei. Most of the dive shops will pick you up using a long tail boat. Diving is OK - mostly around the offshore limestone pinnacles with max depth about 18-20 metres.
Local diving around Phuket is similar - there are a few sites around a 5-10 minute long tail ride away. The main dive sites are reached by dive boat from Chalong beach on the South East side of Phuket - sites like Anemone Reef, King Cruiser wreck, Shark Point, Koh Racha Yai and Koh Racha Noi. They are usually within 1 to 1 1/2 half hours away.
Many of these sites are midway between Phi Phi and Chalong. Phi Phi also has local diving again around the limestone pinnacles and generally this is of a higher quality than Krabi since the visibility is much better.
You can shop around the dive shops in Phuket and Krabi but on Phi Phi there is a cartel arrangement - they all quote the same price. The main bit of Phi Phi is very touristy and crowded. There are some hotels at the North East end of Phi Phi which are much quieter but you have to get a long tail to get to the main area where all the dive shops are - there are no roads on Phi Phi.
Best diving in the area is Hin Deng/Hin Muang which is South of Krabi and South East of Phi Phi - this would usually be an overnight trip since its a few hours by boat - Phi Phi is closer than Krabi.
Hope this helps
Regards
Welshman