You didn't mention if it's for Illuminations or Wishes. Anyway, they usually want you at the boat about 45 minutes ahead of the show time for the fireworks. Pushing it would be a 1/2 hour before the show. You take a cruise around the lagoon area first then they will park it for the show.
When you make your ressies the CM can tell you the exact details of how soon to be at the dock area. They may not know October hours yet, either. You may have to wait til October hours are released to know for sure. Usually, it's best to be on the safe side and plan your meal early even it means a little dead time in between dinner and the fireworks cruise. Dinner could run late with seating, etc. If you need to go to a different location from eating to cruise that takes time, too.
The biggest worry is getting a cruise ressie for FW especially for Illuminations. It's not that easy. Wishes has more boats; so, this is easier. Good Luck.
