Ask when to charter in Thailand and the answer comes back with unusual precision: November to April. That is the dry northeast-monsoon season on the Andaman coast, and it is when Phang Nga Bay is glassy, the outer islands are reachable, and the Similan and Surin national parks are open. But the six months are not identical — the water, the crowds and the prices all move within them. This is what the season actually means, month by month.
What the northeast monsoon changes
From roughly November to April the prevailing wind on Thailand’s Andaman coast swings to the northeast, which is offshore for most of the cruising ground. The sea flattens, visibility improves, and the passages west to the outer islands become comfortable rather than committing. In the opposite half of the year the southwest monsoon pushes swell straight onto the same coast; charters still run, but they keep to the sheltered eastern bays. The season is not a marketing convention — it is the wind direction.
The season is not a marketing convention — it is the wind direction.
November — the season opens
November is the turn. The rain tails off, the sea settles, and the bays are still comparatively quiet before the high-season influx. Visibility is improving week by week rather than at its peak, and there can be a residual swell early in the month. For guests who value empty anchorages over guaranteed glass, it is one of the best-value windows in the calendar.
December to February — the peak
These are the settled months: dependable sunshine, the calmest water of the year and the best visibility for diving and snorkelling. They are also the busiest and the most expensive, and the Christmas and New Year weeks are booked far in advance across the whole fleet. If your dates fall here, the constraint is not weather but availability — the better vessels for the festive period are reserved many months ahead.
March and April — warm, clear, and quieter
The tail of the season is a favourite among people who charter Thailand repeatedly. The sea is warm, conditions remain settled, the peak crowds have thinned, and April in particular can offer excellent water with noticeably more space at the anchorages. It grows hotter and more humid as April goes on, and the first signs of the changing monsoon usually appear towards the end of the month.
The national parks, and what that means for a route
The Similan and Surin islands — the finest diving on this coast — are national parks, and they are open during the November-to-April window and closed in the low season. This matters for route planning: a charter that includes the Similans is only possible inside the season, and those passages are the ones that most benefit from settled northeast-monsoon conditions. Phang Nga Bay, Phi Phi, and the Racha and Coral islands are cruised year-round, but they too are at their best in these months. Park, ranger and cruising permits are arranged by the crew and the desk.
And the other half of the year
May to October is wetter and windier, with sailing concentrated in the sheltered bays east of Phuket. Prices fall and the bays are quieter, and a well-planned charter in the low season can be genuinely good — but the outer islands and the Similans are effectively out of reach, and the weather asks for flexibility. If your dates are fixed to those months, the desk will tell you honestly what is realistic rather than sell you the high-season itinerary.
