North along the chain to the little diving island - a morning catamaran puts you under water by early afternoon.
Find a room before you go.
Lomprayah's morning catamaran from Maenam calls at Koh Phangan and reaches Koh Tao before noon - fast enough that dive schools schedule afternoon boats around it. The stretch beyond Phangan is open water: pick the big catamaran and a calm-season date if you're a nervous sailor.

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The standard: Maenam to Mae Haad with a Thong Sala call. Buy return legs together in high season.
Songserm's slower boat saves a little and takes longer - fine on glassy days.
Many travellers ride Samui → Phangan → Tao over a week or two - tickets split into legs at no real premium.
Step-by-step transport guides for this stretch of the Gulf coast.