Beach Pictures Thailand
Diamond beach, Ko Samet, in December.
Thailand's beaches are well established, have warm water and weather, good food, friendly relaxed people, comfortable accommodation and they're cheap - incredibly cheap when compared to the Caribbean, Indian Ocean or Pacific island paradises. Around $30/£20 a day should get you an excellent double room with aircon, breakfast and maybe even a swimming pool, though Phi Phi Don and Phuket prices will be slightly higher than other beaches listed here.
These beaches do have a clear best/worst time to visit - see Thailand Beaches Information to help you there. They also have less apparent regional flaws that you can see exposed in the different sections of the Beach Pictures gallery.
Phuket and Ko Samui are the only beach areas with international airports, which makes them convenient but crowded - particularly with packaged people. However, Thailand's roads are generally excellent and transport is comfortable and reliable, so a flight to Bangkok - and anyone with a grain of world curiosity has to see this wacky city - followed by a bus/minibus to the beach of your choice is no hardship.