I visited samet Island 3 times and i think the best beach is Hat Sai Kaeo. Diamond Beach has different bungalows and a good restaurant. It is a place prefered by thais. The prices are reasonable, i cannot pay like 200§ per a day, i pay about 600 Baht (!). For going there from Bangkok is possible by minibus and boat, but be aware, that some company bring you not to Ban Na Dan of samui, instead to some beach and you must stay at this beach. The best way to go is from Pattaya to Ban Phe. On Ban Phe take a regular boat to Ban Na Dan. If you don't like to stay in Pattaya, you can take a regular bus from Bangkok to Pattaya from Eastern Bus Terminal in Bangkok and a minibus from Pattaya to Samet.But it is better to arrived Samet on Morning for looking a bungalow. Weekends and public holidays and scool holidays Samet is mostly overbooked. But this time maybe you can meet same nice thais. For the trip take not to much baggage with you! And beware, that the costs of moneychanges are higher.