I suggest you to choose Sihanoukville, is known to have some of the best beaches in Cambodia.
Located in the center of a small peninsula that extends into the warm waters of the Gulf of Thailand, Sihanoukville boasts several fine white sand beaches.
The main attractions of Sihanoukville are its beaches and islands, and scuba fans will delight in underwater excursions around Sihanoukville’s outlying islands.
The beaches of Cambodia don’t measure up to the serene beauty of the beaches in Thailand. Nevertheless, Sihanoukville is a a popular choice for beach lovers and a popular hangout with the backpacking crowd. Being a short drive away from the capital city of Phnom Penh, the best beaches in Cambodia can easily be reached in a few hours by bus or private car.