Phulay Bay and Rayavadee. We split our time there last year. Both were wonderful. Railay Beach, location of Rayavadee was beautiful, though we did go when there were less crowds.
Actually, another option to get from Phuket (we stayed at Como Point Yamu - pool villa was very nice and this resort has a modern, earthy vibe to it) to Krabi is by land. We had Rayavadee pick us up from Como. The drive was effortless: they supplied snacks, drinks and WiFi in their hotel's vehicle. Two or so hours later (nice countryside views on the way) we were whisked to the island on their boat.