Koh Samui island sits in the Gulf of Thailand off the country's east coast — not in the Andaman Sea, where Phuket and Krabi are. That one line explains most of the rest: the calendar the island runs on, and why you cannot drive here.
What follows is a short orientation: where the island is, what the water around it does, how people arrive, how the four coasts differ and what actually runs on the water. Every number comes from the operators, the airport or named climate archives.
- Where — the Gulf of Thailand, with the airport in Bophut on the north coast.
- Getting here — about 50 minutes by air from Bangkok, or the Raja car ferry from Don Sak at ฿200 ≈ $6 per adult.
- When — dry in February, March and April; November is the wettest month of the year.
In short
Samui is the Gulf, not the Andaman, so the wet season falls in October to December. Sea days leave mostly from Bangrak pier on the north coast, a day in Ang Thong Marine Park starts at ฿2,300 ≈ $64 per person, and from 1 November to 15 December the park is shut outright.
Where is Koh Samui island?
Samui is Thailand's third largest island. It lies in the Gulf of Thailand roughly 700 kilometres south of Bangkok and some 35 kilometres off the mainland coast of Surat Thani province. The nearest mainland port is Don Sak, and that is where the car ferry leaves from.
Administratively the island belongs to Surat Thani province and is divided into sub-districts; the airport stands in Bophut on the north coast. Two neighbours are linked to Samui by daily sailings: Koh Phangan to the north and Koh Tao further north again.
Which sea surrounds Samui?
The Gulf of Thailand. It is shallower and calmer than the open Andaman Sea, and the swell is noticeably lower: from May to October there are on average no days at all with waves over a metre, while from November to January there are eight to ten a month.
The water is warm year round and barely enters the decision about which month to come. The coldest month is January at 27.9 °C, the warmest is May at 30.5 °C. The gap between the best and worst month of the year is two and a half degrees.
Getting onto the island
There is no bridge to Samui. Flying is fastest: Samui Airport itself gives the flight time from Bangkok as about 50 minutes. It opened on 25 April 1989 as Thailand's first private airport, and its 2,100-metre runway takes narrow-body aircraft of the A319 and Boeing 737 class.
The ferry is cheaper. On Raja Ferry's Don Sak to Lipa Noi crossing an adult ticket is ฿200 ≈ $6, the passage takes about an hour and a half, and departures run hourly from 05:00 to 19:00, fifteen sailings each way. Cars and motorbikes travel only on this ferry, never on the high-speed catamarans.
The coasts: where you end up
Samui has four distinct coasts. The east is Chaweng and Lamai, the busiest beaches on the island. The north is Maenam, Bophut, Bangrak and Choeng Mon, and most sea days leave from here. The west is Nathon and Lipa Noi, which face the mainland and take the ferries.
Bangrak pier on the north coast wins on drive times: it is about 10 minutes from Chaweng, 11 from Bophut and from the airport, and 33 from Lamai. Kayaks to Ang Thong leave from Nathon on the west coast at 08:00, and the dive boats go from Bophut.
What people do on the water here
The island's headline sea day is Ang Thong Marine Park: forty-two islands an hour and a half to the west. The speedboat day starts at ฿2,300 ≈ $64 per person plus the ฿300 ≈ $8 park fee on the day, eight hours from Bangrak pier with pickup at 08:30.
The same park can be taken slowly: the sea-kayak day starts at ฿2,800 ≈ $78 per person, eleven hours from Nathon pier, with the marine park fee and the transfer already included. If you only want the water, a snorkeller's seat on the dive boat is ฿2,900 ≈ $80 with lunch.
The rest is about the shore and about speed. Four hours on a jet ski around the island is ฿6,500 ≈ $180 per machine for two, a kitesurf discovery lesson at Hua Thanon is ฿7,000 ≈ $194 for three hours with a private instructor, and a sunset sail is ฿22,000 ≈ $609 for the whole yacht.
The neighbours: Phangan and Koh Tao
Both neighbours are reachable from Samui in a single day. The Koh Tao and Koh Nang Yuan speedboat day runs 8.5 hours from Bangrak pier with pickup at 08:00, at ฿2,300 ≈ $64 per person plus the Nang Yuan entry of ฿250 ≈ $7 per adult and ฿120 ≈ $3 per child.
If the point is to go under, Koh Tao has a try dive: two dives to 12 metres, a maximum of two students per instructor, ฿5,900 ≈ $163 per person, no certificate needed. Certified divers head for Sail Rock instead, from ฿4,650 ≈ $129, pickup at 07:45.
When to come to Koh Samui
February to April. February averages 4.3 rainy days and 64.3 mm of rain, March 4.7 days, April 6.1. For comparison, November has 16.5 rainy days and 444.6 mm — more than February, March and April combined.
Important
Ang Thong Marine National Park is closed from 1 November to 15 December every year. That is not a weather caveat or one operator's decision — for those six weeks nobody runs trips into the park.
