We missed St. Maarten in October 2000. There was a storm in the eastern Atlantic, so we called at Key West on the first day. By the end of the day, the storm had downgraded and turned south, so we spent the next two days at sea and called at St. Thomas on Wednesday as scheduled. Except for a bit of rain on Sunday morning in Key West, the weather was perfect for the entire trip, including
Castaway Cay.
Bear in mind, that you could still get diverted, it's more likely to happen at this time of year, but it is still unlikely. We sail in the fall every year, since the prices are so much lower than during the peak times, and have only been diverted the one time.
Based on the trip reports on this site, the Magic is 11-12 in October calling at Castaway Cay, that is, of the trips I can account for, it's only missed once, in Oct. 2001. The Wonder missed in October last year, but is 20-21 in October. The odds are overwhelmingly in your favor to make it to Castaway Cay.