Patrick, yes, you can dive from the Gulf islands, particularly Tao, in September. You can also dive from Phuket. You only can't go to the Similans. We dive year round from Phuket (as well as from Tao), and we also have some low season liveaboards out to the southern islands and the Hin Daeng area during our low season. There's no need to go to Indonesia, though you certainly could do so. I love Bali in September, and Lembeh is a joy at that time of year, too, if you like the sort of diving that's available there.