It is suggested to buy boat tickets in advance using websites of the boat companies bellow.
Surat Thani (Donsak) to Koh Samui
Both Raja Ferry and Seatran Ferry have service every hour starting from Donsak Pier at 05:00, with the last one departing at 19:00 pm. Service is available for both people and vehicles and takes about 1:30.
Lomrayah usually has two trips – at 11:45 and at 14:30, it takes only 45 minutes, but the price is much higher (at the moment 170 Baht vs 450 Baht). And it is not possible to travel by vehicle. Trip is starting from Donsak as well.
Koh Samui to Surat Thani (Donsak)
As for return trip the situation is the same. Raja Ferry and Seatran Ferry have service every hour starting at 06:00 am, with the last one departing at 07:00 pm. Service is available for both people and vehicles and takes about 1:30. Raja Ferry starts from Raja Ferry Port in Lipa Noi. Seatran starts from Nathon Pier.
Lomrayah has two trips also – at 08:30 and at 11:45 from Nathon Pier, it takes only 45 minutes, but the price is much higher. And it is not possible to travel by vehicle.
Chumphon
It is possible to go to Koh Samui and back from Chumphon. Lomprayah provides combined tickets to go there from Bangkok with an ordinary or VIP bus. On Koh Samui Lomprayah operates from Pralarn Pier.
Departure times from Bangkok are 06:00, with arrival at Koh Samui at 16:30 and at 21:00 with a night trip and arrival at 11:30 the next day.
The back trip from Koh Samui starts either at 08:00, with arrival at Bangkok at 20:30, or at 12:30, with arrival at 00:45 the next day.
The speed catamaran starts from Thung Makham Pier in Chumphon at 07:00 and 13:00 with a travel time of 04:30. It arrives back from Koh Samui between 11:45 and 16:30.
At the moment, the price for one-way travel via boat is 1200 Baht, and it increases to 1450 or 1750 if combined with an ordinary or VIP bus.