From the quiet beach to the diving island in half a day — a two-hour train and a catamaran, timed like they planned it.
Find a room before you go.
Take an early southbound train to Chumphon, transfer to the pier, and the catamaran lands you at Mae Haad in the early afternoon — checked in and snorkelling before sunset. Buy the boat ticket ahead in high season; the pier transfer is included with combined tickets.

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About two hours on the rails and under two on the water with Lomprayah's catamaran. The morning connection is the one to aim for.
Evening train down, midnight slow boat with mattresses on deck, sunrise arrival at Koh Tao. Cheap and oddly magical.
Drive to the Chumphon piers (secure parking available) if you want your car waiting on the mainland for the trip home.
Step-by-step transport guides for this stretch of the Gulf coast.