One catamaran, two islands: the northern sea road to Phangan calls at Koh Tao on the way - stopover strongly encouraged.
Find a room before you go.
Lomprayah's catamarans continue from Koh Tao straight to Thong Sala, making Chumphon the natural overland gateway to Phangan from the north. If your schedule allows, break the trip on Koh Tao - the two-island combination is the Gulf at its best.

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The standard and the scenic: under two hours to Koh Tao, a short call, then an hour more to Phangan across open blue.
Tickets are cheap enough to split - a night or three of diving on Koh Tao before the second hop south costs little extra.
The same boats continue to Samui - Phangan-first itineraries slot neatly into a three-island loop ending at Samui's ferries or airport.
Step-by-step transport guides for this stretch of the Gulf coast.