I've sailed on Anthem in March when there was snow on the ground, and I sailed October last year. In Sept/Oct you shouldn't have to worry about the weather too much. The first day sailing out might cool outside especially at night but I wouldn't say it would be a waste. But then it could also be sunny and hot with this north east weather!
Anthem is a ship that seems designed to be able to handle cold weather, worse case scenario there's an indoor pool to enjoy if the outside weather isn't quite warm yet. If possibly wearing a light sweater or light jacket the first day won't ruin your trip, I think you'll be fine!