Destination: Thailand

A touch of apprehension tempered my excitement as the boat approached Richelieu Rock, an isolated seamount in the Andaman Sea about 25 miles off the west coast of Thailand. Last night's full moon had brought some wicked currents to these waters, and it occurred to me that a hapless diver lost at an inopportune moment in the cycle of tides might drift for days before finally washing up on the remote shores of southern Burma.But when we stepped off the stern and began our descent along the mooring line, I was confident that the boat's Thai skipper and Swiss dive master had nailed the timing perfectly. Only the slightest hint of current wafted over the submerged summit, and conditions were blissfully clear as we reached a maximum depth of 90 feet and embarked on a leisurely counterclockwise circuit of the granite pinnacle.For the next 45 minutes, we were privileged guests in a vibrant vertical ecosystem bustling with abundant populations of hard and soft corals, fish and invertebrates. Orange and white clownfish cowered among the fluttering fronds of sea anemones. A green moray stared unblinkingly from its protective crevice. Nearby, in the shadow of a big cabernet-colored sea fan, a pair of delicately dangerous lionfish hovered side-by-side with gossamer fins and poisonous spines extended. Richelieu Rock is best known in Thai scuba circles as a likely place to see whale sharks, especially from February to April when divers report frequent encounters with the massive yet harmless filter-feeders. In Phuket, the epicenter of tourism in southern Thailand, there is a diverse fleet of liveaboards. While several excellent sites are accessible on day trips including the islands of Koh Phi Phi and Koh Racha, and the King Cruiser, a 275-foot car ferry sunk in 1997. Serious divers should consider longer trips to the Similan Islands, Koh Bon, Richelieu Rock and the Surins. Excursions along this string of mostly uninhabited islands offer easy access to many exquisite sites, with short cruising times from one site to the next. Longer itineraries travel to more great dives in Thailand's deep south, or as far north as the previously forbidden and relatively unexplored waters of southern Burma.High season in the Andaman Sea is from November to April, when reliable sunshine and calm seas mean that divers seldom have to wait on weather. Day diving is possible year-round, but from May through October when the southwest monsoon brings questionable conditions to the west coast, most divers head east to the Gulf of Thailand. Although the gulf offers fewer top-notch dive sites, the island of Koh Tao has developed a solid reputation among divers, and some excellent sites are accessible from nearby Koh Samui and the sprawling resort town of Pattaya near Bangkok.Whichever sea you choose, water temperatures average a comfortable 85 degrees year-round. And you're never far from Thailand's terrestrial attractions: Bangkok and the rich cultural heritage of the central and northern provinces; lush tropical rainforests in the south; and beautiful beaches on both coasts where accommodations range from $5 backpacker bungalows to posh five-star resorts.