I take a sip of my fresh ­coconut water and exhale. It’s hard to believe I’m on Koh Phangan, the Thai party island notorious for its raucous backpackers’ rite of passage: full moon parties.

Photos of the raid distributed by police showed a dark, crowded room with casually dressed partygoers, almost all wearing face masks. Koh Phangan in Surat Thani province is a popular destination ...