Hi janfred. I have not departed from Cape Liberty, so I am not sure exactly what you'll see from the ship. As far as weather in late September... anything goes. According to weather.com, here are some average low and high temperatures for the last week in September, keeping in mind the lows are typically recorded overnight.
Quebec City 41-60F
Bar Harbor 45-66F
My best tip, after several C/NE cruises, is to be out on deck to watch the approach into Portland... the most beautiful I've seen in any place outside of Norway, with its combination of beautiful scenery and numerous lighthouses.