Thailand has a tropical climate with three distinct seasons. If you're looking for maximum time on the beach, plan your visit from March to the end of April, when it's hot and dry with average temperatures of 82 to 91 degrees Fahrenheit. May to October is a mix of rain and sun, with average daily temperatures of 80 to 86 degrees, while it's a comparatively cool 75 to 80 degrees from November to February. 
Beaches on Bangkok's Doorstep
Most people will fly into Bangkok, one of the most exciting cities in the world. Pattaya, a famous international seaside playground 87 miles to the southeast of the capital, has been a rockin' beach resort for more than three decades. Here you'll find plenty of hotels, restaurants, boutiques and nightclubs that cater to an international clientele. 
Sophisticates are making their way to Hua Hin, a seaside resort that's a three-hour drive from Bangkok. Even though Hua Hin lays claim to being Thailand's oldest beach resort, it's now a decidedly trendy haven for the well-heeled, like Thailand's version of the Hamptons. In addition to watersports, visitors to Hua Hin can ride horses, play tennis and tee off on a number of golf courses.
Further South Along the Shore
Southern Thailand extends along a narrow peninsula lying between the Andaman Sea on its west side and the South China Sea on the east. Its long coastline of sandy beaches and offshore islands offers an embarrassment of riches. Here you'll find some of the most famous seaside resort areas in the world, such as Phuket, Koh Samui and Krabi. 
The east coast on the Gulf of Thailand presents calmer seas and a laidback atmosphere, while the Andaman Sea coast is more often used as a memorable backdrop in movies. Fans of James Bond will remember the limestone rock formations and cliffs featured in The Man With the Golden Gun.
Phuket is Thailand's largest and most visited island, made vivid by an exotic setting of coconut groves and rubber plantations. Phuket is easy to reach, connected to the mainland via the Sarasin Bridge. Phuket's busiest beach is Hat Patong. Here you'll find the largest selection of hotels and restaurants, and the most vibrant nightlife. Phuket's longest beach is Hat Mai Khao, while Hat Rawai is a palm-fringed beach best known for its community of sea gypsies—a formerly nomadic fishing group of Melanesian descent. Thai Watersports
Koh Samui is an island off the east coast of Kra Isthmus, famous for its beach raves. Its sandy bays are lined with bungalows and resorts, as well as plenty of restaurants and hip nightlife venues. Hat Chaweng and Hat Lamai are Ko Samui's most beautiful and most popular beaches. Koh Pha-ngan—Koh Samui's neighbor island—is notorious for hosting the world's biggest beach full-moon party.