Thailand in October?

No one can predict if your going to get rain or not. The monsoon season has not started in the southern gulf region until around Dec so you will have no problems in Koh Samui in Oct but you could get some rain like everywhere in Thailand , I was there during that period a few years ago and it was great. As for the other side it's the beginning of the end for the rainy season. Again you may see some rain but it's not going to stop you from enjoying yourself at the beach. Its could only be a short heavy rain for about 30 minutes.