Tropical climate. Rainy, monsoon season (May to November), dry season (December to April). Little seasonal temperature variation. Bring your boardshorts and/or bikini! Maybe a rash vest for sun protection. It will never be too cold.
From Phnom Penh take a bus from the Central Market to Sihanoukville about 5-6 hours. Then grab a motodube (motorbike taxi) from the Sihanoukville bus station and go to the beach. Located west of Sihanoukville.