Concierge.com's insider take:
The most serious ragers use Koh Samui merely as their jumping-off point to the nearby island of Koh Phangan, home of the legendary Full Moon Party. Tens of thousands of bikini- and tie-dye–clad revelers drenched from sweat and seawater keep Asia's most outrageous party hopping after dusk. Expect to cheer the sunrise as boats pull up to take exhausted revelers back to their resorts. If you can't make it on the actual day of the monthly Full Moon Party, you're likely to catch similar levels of debauchery on the two days leading up to and after the actual event. Be forewarned that recreational drugs are common, despite their being illegal in Thailand; use at your own risk. The Samui Phangan Express leaves Samui every hour on the hour between 8 am and 5 pm for the 12-minute ride to the rave (Tel: 66-81-372-701; U.S. $15 one way). People arrive well before sundown and stay until the boats start leaving again in the morning.