The Andaman coast is usually best from November to April, when seas are calmer and offshore visibility improves. This is the main window for trips from Phuket, Krabi, and Khao Lak, including the Similan-Surin circuit accessed via Khao Lak for the Similan Islands. Offshore water often sits around 26°C to 29°C in season. May to October can bring monsoon wind, swell, and cancellations.

Koh Tao is often most consistent from March to September, when seas are calmer and day boats run reliably. Water commonly sits around 27°C to 30°C, and good spells can deliver visibility around 15 m to 30 m. February to April is popular for sunny weather, and spring can bring better odds of whale-shark sightings on offshore pinnacles. Later in the year, rougher seas and heavier rain can disrupt ferries, so keep buffer time.

Use Thailand's flights to chase conditions. A common plan is Gulf first for training and repetition on Koh Tao, then fly via Bangkok to the Andaman for offshore reefs via Phuket or Khao Lak. This reduces the risk of getting stuck on one coast during a monsoon pulse. Keep at least one buffer day around big ferry transfers, and confirm marine park opening dates if your Andaman segment targets Similan-Surin trips.

These Andaman marine park areas usually operate seasonally and often close during the southwest monsoon for safety and reef recovery. In many years the closure runs roughly mid-May to mid-October, with reopening around mid-October, but dates can shift. If your trip depends on the circuit accessed via Khao Lak for the Similan Islands, confirm official opening dates before booking a liveaboard or day trip. Even in season, exposed routes can cancel in rough weather, so keep flexibility.

TDAC is an official online travel information submission. It is not a visa. Official guidance typically asks you to submit it within 3 days before arrival. Complete the form online, then save the confirmation (screenshot or PDF) for airline check-in and immigration. For multi-stop dive trips, use your first entry flight and first accommodation details, and keep your onward plan handy in case an officer asks. Requirements can change, so rely on the official TDAC site and your embassy guidance.

Thailand has national rules for diving and snorkeling activities effective 22 April 2025. Visitors should expect clearer supervision standards, safety briefings, and stricter coral-zone behavior. Snorkelers near coral are generally required to wear a life vest unless they hold a recognized freediving certificate. Divers should plan for a no-touch approach: no standing on coral, no feeding fish, and no harassing marine life. Choose operators that actively manage buoyancy and group behavior, and carry proof of certification where relevant.

Many popular sites sit inside national parks, so you may pay an entry fee on top of your boat trip. Similan-area trips often cite around THB 500 per foreign adult, and Phi Phi area fees are often quoted around THB 400 per foreign adult. Some islands add separate maintenance fees (Koh Nang Yuan near Koh Tao is commonly THB 100-250). Fees can be bundled by operators or collected by park staff, and rates can change, so confirm the latest price before departure.

Thailand is beginner-friendly thanks to warm water and lots of instructors. Koh Tao is the classic choice for course volume and repetition, with water often 27°C to 30°C. Phuket is a strong alternative if you want a larger island with more topside options and easy international access. A smart combo is to certify in the Gulf, then do fun dives on the Andaman side in the clearer season. Pick a shop that prioritizes small groups and buoyancy skills.

For easy shore and short-boat snorkeling, Koh Tao is hard to beat because many bays are sheltered and shallow. For organized day trips with simple hotel pickups, use Phuket or Krabi in the Andaman high season. For a quieter, beach-first escape, Koh Lipe and the Tarutao area can be excellent when seas are calm. Aim for mornings, and follow life-vest rules near coral zones.

Thailand has established hyperbaric and dive medicine support in major tourist regions, but transfer time still matters from remote islands. Phuket supports the Andaman coast with hospital-based diving medicine and hyperbaric oxygen therapy. The Koh Samui area supports the Surat Thani district, including Koh Samui, Koh Tao, and Koh Phangan, through dedicated chamber networks. Bangkok hospitals also provide hyperbaric services for referrals. Carry dive accident insurance, save emergency numbers, and report symptoms early so your operator can coordinate care quickly.