Paris in December

I don't think you'll need boots. Most of NW Europe averages well above freezing in December. If you see any snow at all, it will be a very light coating that quickly melts. It does rain frequently, but not in torrents. Usually only in light sprinkles. And in Paris, you will probably spend a significant amount of time indoors. Your feet may become uncomfortably warm in winter boots. Finally, all the Rhine cruise boats that I see in operation during the winter months are mostly enclosed, so even there, you will be inside for long periods of time. Here's what I would do- get some warm pairs of socks. The weather where I live is about the same as in Paris (maybe slightly cooler), and I wear pretty much the same shoes in winter as I do in the summer. The only thing I change is if I'm going to be outside for long periods of time, I wear thicker socks. My feet are never cold or wet, and I generally spend a lot of time outside even in the winter. The only place I ever need to wear winter boots is in the Alps, where I know the snow will be deep and the temperatures usually stay well below freezing. Now of course, if you wanted to wear boots purely for style, that's a different matter, but not one that I can particularly help you with.