September is a great time to cruise, but don't tell anyone. The rates are low because people don't want to cruise because they are afraid of huricanes. If every one finds out how great it is, the prices will go up. So keep it a secret. Maybe this post will self destruct after you read it.
We took our first ever cruise in September 2000. We liked it so much that we cruise again in September 2001 and have another one booked for September 2002. If there is any chance of a huricane in the vicinity of the ports, they re-route elsewhere. So as long as you don't have your heart set on the specific ports, you should have no problem. I have heard from people who have cruised both in September and December, that the seas where actually rougher in December.
Good Luck any way you go.