The Gulf's shortest island hop - twenty-odd minutes of blue and you trade beach clubs for beach bonfires.
Find a room before you go.
Ferries shuttle between the sister islands all day from several Samui piers - most days you simply pick the next departure. The exception is Full Moon, when dedicated party boats run late into the night from Bangrak and everything else sells out by lunchtime.

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The quick hop - about 20-30 minutes to Thong Sala. The natural choice from Samui's north coast.
Slower but takes scooters and cars - rent on one island, ride on both.
Round-trip party boats from Bangrak sail into the small hours on the big night - book the return leg with the outbound.
Step-by-step transport guides for this stretch of the Gulf coast.