I went in Jan/Feb of this year so I'm not sure what you should expect in Oct. If I remember correctly, the crew told us diving was better during the fall. I always enjoy my dives anywhere, especially in a new environment (I've dived the Caribbean, but mostly in the western part).
Expert to see Caribbean reef sharks (cool) and LOTS of red lionfish (more there than I saw in the Philippines where they are native). However, I was a little disappointed to see many of the species I've observed in the western Caribbean were significantly smaller in the Bahamas and much less plentiful. Observations like this just get me thinking why things were different there than elsewhere in the Caribbean... hurricane impacts? lionfish? sea urchin disease?
I enjoyed the diving. Night dives were great. Enjoyed filming turtles which are rarely seen here in SoCal. The crew on our trip was great with the exception of the DM (who was still tolerable), but I don't think she is still with them. You should have a good time.
