I pretty much agree - it's the tropics. I've been in September, March, May, I've never been when I didn't get at least a little rain. I really prefer September because things are much less crowded. In my experience, it's hotter, but generally less windy. I don't know if the few weeks' difference from Sept to Oct will matter or not. This year has seemed to be more rainy than normal, but someone who lives there could probably better comment on that. I'd be really surprised if it rained the entire time no matter when you went.
Have a great trip!!