Despite the many confusing ideas TripAdvisor might put in your head, the cheapest way to get from Bangkok to Koh Tao is using the Lomprayah service. The ticket costs 1050baht per person and includes the bus and high speed ferry. We looked at taking the night train and then the ferry. This would be far more comfortable, however more expensive. There is a combined train and ferry ticket for 1240baht per person (2nd class sleeper with AC). To save travel agent commissions, you can buy the ticket by yourself at the Lompraya office at the below location.
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There is an early morning bus and a night bus. We took the night bus, which is not a sleeping bus, so get ready for an uncomfortable 7 hours sitting up. The best seats are on the top level on the right side, but these are highly sought after and relatively randomly assigned by the ushers. The bus stops once, however there is a toilet on board. The journey takes you to Chumphon Pier, not Surat Thani. Read how to get the night ferry from Surat Thani to Koh Tao if you are coming from the South…
We left on the day of the King’s birthday, so our bus was 3 hours delayed, however I understand that normally you would leave at 9pm and arrive at 4am, where you wait for 3 hours until the first ferry departs. We arrived around 6am and lined up to get another sticker to put on our shirts. Then we waited another hour until they opened the ferry for boarding at 7am. It takes about half an hour for everyone to board. If your bags are small, keep them with you. Placing them in the luggage heap, means you will join the mad, disorganised scramble on the tiny pier on Koh Tao to get your bag back. In December the seas can be rough and they certainly were. The journey to Koh Tao takes about 2 hours and 1 hour 45 minutes of those will be spent cowered forward vomiting into black plastic bags that the ushers hand out. They are very nice about it, but of course it must be quite amusing to them. The smell is hard to describe and will most likely make you vomit even more. The first stop is Koh Nangyuan, then Koh Tao. After that the ferry continues to Koh Phangan and Koh Samui. Once you arrive at Koh Tao and have somehow magically found your bags again, walk straight up the pier through the ferry terminal and then turn left and right when you see a 7-11. This is the main road. Then regroup. Ignore all touts, especially taxis, they are there to rip you off. You can walk to Sairee beach in about 15-20 minutes with your bags.
Or, you could just buy the ticket now, and save yourself the hassle!