I totally agree that in the high season the diving is better, due to the better viz and the guaranteed excellent weather. But I dont think it is a bad idea to go to the Similans during October. I even had very happy guests this low season, who were able to dive the Similans with no other boats around, and also their boat was nice and quiet. The viz was 20 meters the last months, and most of them had excellent weather. One trip was cancelled though, but that is the risk of the Similans in the low season.
Last October I spent most of the month on the Similans myself, the weather was good, hardly any wind and it was quiet. Not like in the low season, but still not too many divers around. And the viz in October is normally already at 30 meters. Chances of rain are bigger than later in the season of course, but I really think that because of the lack of other divers and boats and the already good viz, a trip in October is definitely worthwhile. Just no manta rays yet.