I spent some time in Ko Samui whilst In was in Thailand and I travelled down there on the Train & Ferry Joint ticket purchased from a travel agent in Bangkok by a disreputable company called Songserm.
You leave Bangkok on the sleeper train and arrive at Surat Thani very early in the morning, you are then collected by Bus and taken to the Pier at Donsak and the Bus takes around an hour and the Ferry between 1.5hrs to 2hours across to Nathon Pier on Samui. It is quite a long trip so I chose the more expensive option and flew back to Bangkok afterwards abandoning my return ticket.
A tip: If you take this trip make sure you take all your valuables and important belongings on your person and never let any valuables in your case go in the hold of the huge Coach they use whilst you are being transported from Surat Thani to Donsak by Bus, the travel company Songserm has a really bad reputation and every week dozens of backpackers get targeted and valuables stolen on this trip as they open your baggage in the hold of the bus and steal stuff. Considering the long journey the best option is to use the Government Bus Services or better still fly instead if your budget can stretch.