I recently spent just over 50 hours in Phuket and did a day-trip to the Phi Phi Islands. You can see the details in my TR in my signature. But, you certainly could fit in the Phi Phi Islands from Phuket on the 13th. We used: http://www.phuketsailtours.com/tour_phi_phi.html for a day-trip. They were pretty good. The snorkeling was nothing special.