best time to visit Phuket and Phi Phi is mid November through to March. The weather is settled though there might be the odd thunderstorm, the sea will be calmer the water clear and the beaches of Phuket safe for swimming. Temperature wise it going to be in the low thirties during the day. It gets dark by 7pm.
There is a new water park by the brand new West Sands development on the north of the island. I gather the hotel on the resort is stiil in the soft opening stage and some parts are still in development.
Hotel wise there are numerous luxury properties ranging from boutique hotels to large resorts depending on your taste. Phuket is a fairly large island sand there are a number of beach areas people stay at. The largest and Livliest is Patong. Karon and Kata are also well developed beach areas but with easy access to Patong. Bang Tao has has a number of hotels in a manicured resort area called Laguna - Sheraton and DUsit resorts and the top end Banyan Tree. The Mai Khao area in addition to West Sands has the likes of Marriott and Anantara. Have a browse round a site like http://www.phuket.com
for some pictures giving a feel for the hotels and beaches - it also has details of hotels on Phi Phi