I've cruised out of New York in winter and it wasn't bad at all. The ocean wasn't significantly rougher than some Florida sailings. It was cold outdoors for a couple of days, but people were still sitting on deck chairs (with a jacket or sweater), walking and jogging outdoors.
Baltimore is a day closer to warmth than New York. Also, if the past continues to hold true, December isn't such a bad month weather-wise in this area. I think you'll be fine.