Koh Samet

I have travelled to many beaches and islands in Thailand, but in my opinion this is the best beach in Thailand. Hat Sai Kaew beach on the Island of Koh Samet is beautiful. The beach is shallow so you can walk a long way out into the crystal clear sea before the water is up to your knees.
How to get there:
Travel to and from Ko Samet is typically via the small local port at Ban Phe on the coast. Travelers from Bangkok frequently travel by bus, with departures to Ban Phe from Bangkok's Eastern Bus terminal, the Ekkamai bus station, and travel time from Bangkok to Ban Phe of approximately 3½ hours. Owing to the island's close proximity to the coast, just 7 kilometres (4.3 mi) offshore, boats travel casually from Ban Phe to Ko Samet in approximately 30 minutes.
To get there quickly you can hire the services of a speed boat that 
will get you there in half the time.
Be aware that there is an entrance fee of 400 THB for entrance to this National Park area.Which is a small price to pay for an amazing beach experience.