Koh Samui weather,
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February to April is peak. Warm, dry, calm sea. Different from the Andaman coast (Phuket, Krabi), Samui is on the Gulf of Thailand with its own monsoon pattern. May to October is actually decent (shorter, lighter rain than the Andaman side), while October to December is Samui's heavy rainy season when the Andaman islands are already turning dry. If you have flexibility, plan around this reversed pattern.

Phuket is on the Andaman Sea (west coast), Samui is on the Gulf of Thailand (east coast). This matters enormously. Phuket's wet season is May to October, dry November to April. Samui's wet season is October to December, dry February to April with a secondary wet period May to June. If Phuket is rainy, Samui might be perfect, and vice versa. For December trips, Phuket is usually drier.

Chaweng is the main tourist beach, longest stretch, most restaurants and bars, busiest. Lamai is slightly quieter with still plenty of infrastructure. Bo Phut is calm, more family-oriented, gorgeous sunsets. Choeng Mon on the northeast is small, calm, resort-focused. Maenam is the quietest with budget options. For first-time visitors, Chaweng or Bo Phut.

In dry season (February to April), yes almost always. In Samui's rainy season (October to December), frequently cancelled due to rough seas. The journey is 90 minutes by speedboat and requires calm water. Book a flexible operator who refunds cancellations.