Sihanoukville is firmly in the southeast Asian tropics — hot, humid, and split into a wet and a dry season.
Dry season (November-April) is the prime time to visit: blue skies, calm seas, easy boat trips to the islands. Christmas and Khmer New Year (mid-April) are the busiest weeks. February and March often have the clearest water.
Wet season (May-October) brings afternoon downpours, choppy seas, and occasional ferry cancellations. The sea can be cloudy and the beaches less appealing. The upside is empty resorts and lower prices. The rain is rarely all-day; mornings are often fine.
The few weeks either side of the rainy season — late April/May and October/November — can be a good compromise.