It's like packing for Alaska. You could have rain, especially with the hurricanes out there that sometimes move up the coast and hit the East coast; it could be cold & it could be nice and warm and sunny. You just can't tell sailing in September.
The best thing to do is dress in layers and make sure you take an umbrella, unless you are in a Deluxe Suite where they are provided.
I'd take at least one pair of shorts (in case it's very warm) and a pair of long pants/jeans for day time wear. Make sure you bring a jacket - doesn't have to be a winter jacket but something that will keep you warm if it's cold/cool.
When in September are you going? Sail7Seas is really the best person to answer this question and hopefully she will see this post before she leaves for the Maasdam tomorrow (I think). She has done this "run" many, many times.
I know I've not been much help, but maybe someone else will have suggestions.
Enjoy your trip.