Phuket is a place that has some good dives at that time of year. Phi Phi island is good for relaxing. The better diving there is the Similans but it closes out in May. Late spring is tough because it is when the rainy season starts or ends depending on which hemisphere you are in. Same thing with late fall. I moved my Bali trip next month up one week because of this; my original holiday destination was going to be Koh Lanta in Thailand. I will be on Bali on 9 November, because it is Southern hemisphere and the start of the rainy season I moved it a week forward. Had I been going to Thailand I would have pushed it back a week. In your case you can shoot for earlier late spring northern hemisphere or later spring southern to get better diving in the regions you are looking at. Have a great time...Tom