Thailand's southern islands split into two clusters that sit on opposite seas. Getting between them requires a crossing overland, so it pays to plan the sequence carefully. Ten days minimum is realistic if you want more than airport-to-airport transfers.
Andaman side — the west coast
Phuket is the natural entry point: an international airport, ferry piers, and everything else within 90 minutes. Base yourself in Kata or Karon for the beaches, in Phuket Old Town for architecture and food, or on the northwest coast around Nai Yang for calm and fewer crowds.
From Rassada Pier in Phuket, ferries reach Koh Phi Phi in 1.5 hours (400-500 THB) and Krabi in 2 hours (450 THB). The Phi Phi archipelago has just one main inhabited island (Phi Phi Don) and its uninhabited sister (Phi Phi Leh, home to Maya Bay). Stay 2-3 nights minimum — day trips barely scratch the surface.
From Koh Phi Phi continue to Krabi mainland (Ao Nang beach and Railay peninsula) in 90 minutes. Railay is accessible only by longtail from Ao Nang (100 THB, 15 minutes) — no roads. Ideal for climbing, kayaking sea caves and short jungle walks to hidden lagoons.
Koh Lanta is 2 hours by ferry south of Krabi (350 THB) or from Phi Phi (1.5 hours). Long, quiet beaches, an old fishing village charm and easier prices than the northern Andaman islands. Perfect for slower travel and families with kids.
South of Lanta: Koh Kradan, Koh Muk (Emerald Cave), and eventually Koh Lipe near the Malaysian border. Ferries operate mostly November-April; low season transport is patchy.
Crossing between seas
There is no direct ferry between the Andaman and Gulf sides — you cross the Kra Isthmus by land. Options:
(a) Krabi to Surat Thani by bus (3 hours, 250 THB) then ferry to Samui/Phangan/Tao (1.5-3 hours depending on operator, 300-700 THB). Combined tickets available.
(b) Fly Phuket to Koh Samui via Bangkok Airways (1 hour, from 3,500 THB return), the fastest but most expensive.
(c) Bus Phuket to Surat Thani then ferry (long day but cheap, around 1,200 THB total).
Gulf coast — the east coast
Koh Samui is the largest and most developed, with its own international airport served by Bangkok Airways from Bangkok, Chiang Mai and Kuala Lumpur. Chaweng Beach for nightlife, Bophut for the fisherman's village and family holidays, Choeng Mon for boutique resorts, Lamai for a middle path.
Koh Phangan is 30 minutes by ferry north of Samui (200-300 THB). Famous for the Full Moon Party on Haad Rin beach — around 15,000 attendees on peak nights, chaotic but legendary. The rest of the island is far calmer: Haad Salad, Haad Yao and the northern Bottle Beach are beautiful and quiet.
Koh Tao is another 1.5 hours north (400-500 THB from Phangan, 600 THB from Samui) and is Thailand's dive capital. Costs are among the lowest globally for a PADI Open Water course (10,000-13,000 THB for 3 days, dorm accommodation often included). Non-divers still enjoy the snorkelling — Aow Leuk, Freedom Beach and Nangyuan viewpoint are all excellent.
Month-by-month recommendations
November-February: peak dry season everywhere, book accommodation 2-3 months ahead. Both sides work.
March-April: still dry, hotter, prices ease slightly. Songkran (13-15 April) is a wet celebration everywhere — plan accordingly.
May-September: Gulf coast (Samui, Phangan, Tao) is generally OK. Andaman side (Phuket, Krabi, Phi Phi) has monsoon rain and rough seas — cheap prices but many boat trips are cancelled.
October-November: transition, unpredictable. Gulf coast can have heavy rain especially November. Andaman side improves week by week.
Booking ferries: use 12go.asia for schedules and one-click booking with real reviews. On-the-ground booths at piers often have same-day tickets 20-30% cheaper. Speedboats are faster (and more expensive) but rougher — regular ferries are more comfortable if you have time.
What to pack: reef-safe sunscreen (mandatory in national parks), dry bag for boat transfers, aqua shoes for rocky beaches, and always cash — many islands have limited ATMs and 220 THB per-transaction bank fees are standard.