Contact more than one travel agent and tell them you're looking for Christmas bargains.
Some agents prefer to work with only one or two cruise lines, so call around for variety.
Keep in mind that certain ports will be quieter around the holidays. If land activities are important to you, look for a cruise in the non-Christian part of the world. Don't just spring the idea on your family. Ask them ahead of time what the most important things about the holidays are to them. Then, when you tell them what you're thinking, point out how they'll be able to have the tree, the presents, the dinner, in a magical night at sea.