I’ve learned the hard way that “best beach in Thailand” depends on one thing more than any Instagram photo: which coast you’re on, and what month you’re swimming. South Thailand’s Andaman side (Krabi, Phuket, Khao Lak, Lanta, Lipe) and the Gulf side (Samui, Phangan, Tao) can feel like different countries in the water, especially in the shoulder months when one coast turns glassy while the other gets choppy or rainy. What follows is how these beaches felt to me on actual swim days: the sand underfoot, the visibility when I put my face in the water, how quickly it got deep, when the longtails started buzzing, and which “spectacular” bays were surprisingly annoying to enjoy.
Andaman vs Gulf: the one decision that changes your whole swim
Before I get specific, here’s the pattern that mattered most on my trips: the Andaman coast is typically at its calmest and clearest in the dry-season stretch roughly November to April, while the Gulf islands often have their calmer window earlier in the year and can take rougher weather later, roughly October to December. In practice, that means a week that feels perfect for a Railay swim can be frustrating on Samui, and a dreamy, flat-water Samui morning can coincide with red flags and rip warnings on Phuket’s west coast.
For planning, I treat South Thailand like two separate beach trips. On the Andaman side, I prioritize sheltered coves, early swims, and keeping an eye on flags in the monsoon months. On the Gulf side, I watch the calendar in late-year months, because even when it isn’t pouring, wind-driven chop can turn “clear turquoise” into milky water you don’t really want to snorkel in.
There’s also a practical season reality: the most famous offshore day trips near Khao Lak (Similan and Surin) have annual closure periods for safety and reef recovery, and those closures can overlap with months people mistakenly plan as their “snorkel highlight.” If your South Thailand dream hinges on crystal water and easy snorkeling, your dates matter as much as your destination.