I'm going on this cruise in September, too from New York to Montreal. Lene, take a poncho or raincoat. You may get lucky and not need it but walking around can be pretty miserable if you don't have it. Umbrellas don't cut it, a poncho like Ann said is the best investment. I always take mine no matter where I'm going. Totes brand sells a great poncho with removable hood and pockets. Trust us!
Susan will be cruisin, too! P.S. I live near the Great Lakes so I know what Sept. can be like.