Hi
Im from Thailand and I know that in August is considered low season in Phuket. It will be difficult to find any tour operator to go to other island as the wave is too strong and will be raining. All the island will be closed during this time. When it rain in Phuket it will rain all day non stop and could continue for several days.
For Samui, there is a chance of having more sunny day however June-November is consider to be rainy season in Thailand so you might have a chance of catching low depression storm.
High season is from November-april as it is more dry and less hot.
