Phi Phi - located in the Andaman Sea in Thailand. The islands are beautiful, with good beaches, and the atmosphere is quite like Ibiza. The only drawback here is a huge number of tourists. However, that doesn't spoil the positive feeling. Beach and seabed: sand. 
All hotels are located on the Big Island called Phi Phi Don. There are lots of accomodation options, but there are also lots of tourists. It is advisable to book room at least one day prior to arrival. In general, prices are higher than on the mainland, but not much.