Thailand is world famous for its diving sites, though most diving is far less impressive than in the Caribbean or South Pacific, including Bali. Ko Tao, in the Samui Archipelago, offers the best diving in the Gulf and serves as a top destination for those looking to get certified.

Popular islands in the Andaman Sea, including Ko Phi Phi and Ko Lanta, boast shallow waters conducive to snorkeling, but only moderately good diving. For the best underwater experience, head to the National Parks of Mu Ko Similan, Mu Ko Surin, and Mu Ko Tarutao, all of which require live-aboard dive boats to access.

Best Bet: Head to the Andaman coast during that coast’s dry season (October to April), when conditions are superb and visibility reaches 100ft (33m). Anyone interested in a certification course can do so through the numerous dive shops; Green Fins Thailand provides environmentally friendly diving guidelines that are practiced by 80 participating dive operators in Thailand (www.greenfins-thailand.org) .

  • Ko Tao: (Surat Thani) - Ko Tao, the northernmost island of the Ko Samui Archipelago, is a world-renowned location for beginners. Diving here doesn’t involve much planning, the waters are calm and shallow, and accommodation is often free if you enroll in a course. Because of the popularity of the top sites around the island (including day-trippers from Samui and Phan Ngan), experienced divers may prefer snorkeling off a long tail boat on a ride around the island.

  • Similan Islands: (Phang Nga) - This collection of nine islands is Thailand’s foremost diving destination. The Similans are just over 60 miles off Phuket, and diving tours are often fully catered, lasting several days and visiting numerous sites around the island cluster. When visitors aren’t exploring the reefs they have opportunity to swim and relax on secluded beaches in this stunning national park.

  • Phi Phi Islands: (Krabi) The Phi Phi archipelago can be reached by ferry from Phuket or Krabi: all of the accommodation is located on the largest island, Phi Phi Don, while the best diving is found around neighboring Phi Phi Lay, Bida Nok, and Bida Nai. The visibility here is quite good and leopard sharks are frequently sighted, but the island hosts hordes of visitors and the best diving will be had from a private long-tail boat, so you can time your dives to avoid the crowds.

  • Ko Lanta: (Krabi) - Ko Lanta is relatively close to the Phi Phi Islands and offers similar diving and snorkeling opportunities. Diving operators tend to focus on the southern tip of the island, is both convenient and less crowded than Phi Phi, but there are many other areas worth investigating. Emerald cave can be reached by boat and is great for snorkeling.

  • Richelieu Rock: (Surin Islands) - The most sought-after site in the Surin Island National Park, Richelieu Rock is best known for whale shark sightings. Manta rays, barracuda, and batfish are a few of the many other species congregating around this 100-foot rock steeple. Live-aboard boats, the only option for diving in the park, can be chartered from Phuket or Khao Lak.

  • Hin Muang and Hin Daeng: (Krabi) - Hin Muang sits far off the Andaman coast, so the only way to reach it is with a chartered boat from Ko Ngai, Ko Lanta, Ko Phi Phi, Krabi, or Phuket. This area features enormous outcroppings of submerged rock, harboring anemones, massive schools of tropical fish, octopus, and several varieties of shark.

  • Ko Kraden: (Krabi) - The wreck site at Ko Kraden can be reached in an hour from Ko Ngai. The ship is a Japanese destroyer sunk during WWII and it plays host to a variety of coral, seahorses, and moray eels.

  • Ko Dok Mai: (Krabi) - The main feature on this rocky island is an expansive underwater cavern. Sea snakes and leopard sharks are the most prominent undersea-life. Ko Dok Mai is equidistance to Ko Phi Phi and Phuket and makes for a good night dive.

  • Shark Point: (Krabi) - Also known as Hin Bida, this collection of rock pinnacles is one hour from Ko Phi Phi. The main attractions here are the six-foot leopard sharks that laze on the sandy bottom.

  • Ko Ma: (Krabi0 - A few minutes off of Ko Ngai, this site is home to a few bizarre species. The most notable is the shrimpfish, which swims upside down by way of small fan-shaped flippers.

  • Marine Conservation Ko Tao: (Ko Tao) - New Heaven Dive Shop, Chalok Baan Khao, Ko Tao • Tel: 66(0)77-457-045 • www.marineconservationkotao.com

    Certified divers can work on coral nursery projects or a sea turtle release program. Diving certification courses incorporate ecological education and underwater clean-ups.

  • Easy Divers: (Samui) www.easydivers-thailand.com

  • Sustainable Marine Adventure and Responsible Tourism: (Phang-Nga) - 56/73-74 Pechakacem Road, Takuapa, Phang-Nga • www.etcth.org

    Thai student-interns work aboard a luxury dive boat, shooting underwater videography for guests experiencing the most beautiful conditions in Thailand.

  • Siam Ocean World: (Bangkok) - Siam Paragon, Siam Square, Bangkok • Tel: 66(0)2-687-2000 • www.siamoceanworld.co.th

    Can’t make it to the islands? Dive into a 100,000 cubic foot fish tank to watch staff feed schools of grey nurse, black-tip, and leopard sharks, as well as southern sting rays and eagle rays.