I won't lie, September is basically the end of the dry season there, so rain isn't the big problem - what ruins it is the rough sea and poor visibility for diving, if that's what attracts you. The prices stay astronomical anyway (the island has always been expensive), but at least in September you've got fewer tourists than in peak months, so you can book things more easily and maybe even get some occasional discounts. Is it worth it? It depends on what you're expecting: if you want calm beaches and perfect diving, you're better off waiting until early in the year; if you basically just want to see the place and don't mind running into some ugly waves, September works and you save yourself a bit of crowds around you.