There are several islands in the Andaman Sea: the east offers extensive reef diving and the west has rocky slopes for swim-throughs. The north end of the coastline is known worldwide for its diverse marine life.
The Andaman Sea holds some of the best coral in Thailand. A shock of purple coral (for which these reefs are known) amid the blue is a treat for divers old and new. Hawksbill turtles, leopard sharks and blacktip reef sharks make appearances as well as grey reef sharks, moray eels, barracuda, octopus, crayfish and shrimp.
Keep your eyes peeled for macro - feathery ghost pipefish are hard to spot, but worth the effort. Night diving in the Andaman Sea reveals an altogether different experience: watch as anemones turn purple and eels glide out from the depths.
Thailand has sun in abundance and it's possible to dive in the Andaman Sea at any time of year. The water stays an extremely pleasant 79-84°F/26-29°C in all seasons. In terms of maximum visibility and calm diving conditions, November-May is the best time to dive.
In March and April, plankton clouds lower visibility but attract manta rays and whale sharks. For fans of turtles, nesting season occurs within October and March, with hatchlings emerging from nests from January to May.
The gorgeous purple coral can be enjoyed all year, but visibility tends to be at its best between November and April (up to 30 meters). If you're hoping for quieter dive sites and a chance to see timid creatures like black tip reef sharks, consider visiting after January when the area is generally less crowded.
For diving in the Andaman Sea, you will need to get to the west coast of Thailand. Most international visitors will arrive into Thailand via the international airport in Bangkok (Suvarnabhumi Airport). From there you can fly to Phuket (1h 25m) - the west coast's main airport. You fly in over the sea and can be on the beach in a couple of minutes after exiting the airport.
Once in Phuket, ferries are the best way to island hop. The "main islands" (Koh Lanta, Koh Phi Phi and Phuket Islands) have well-established transportation systems and are most likely where your accommodation will be. The smaller islands (Koh Rok Nai, Koh Mook) are best suited to day visits and can be reached by dive boats.
90km separate Phuket from Koh Lanta and there is a range of boat services that cover this route - a ferry from Phuket to Koh Lanta takes around 1h 15m. If you have a car, Phuket is linked to the mainland by a bridge and Koh Lanta is served by a vehicle ferry.