I'm not a big fan of Langkawi, so I'm probably not the best person to recommend it. The beaches aren't worth going out of your way for in my opinion. The water, while clearer than some other West Coast areas, still isn't that clear. Phuket has a similar weather pattern, as it's in between wet and dry seasons. However, I think the weather should be better, but you never know.

Have you thought about Phu Quoc, Vietnam? The weather should be better in October. it's a non-stop flight. I haven't been in almost 15 years, and that was prior to the airport, so I'm sure it's much more built up.