Planning a med cruise in December. What is the weather during that month? Are the seas smooth then? Does it rain much?
Yes, it can rain a lot and yes, sea could be rough.
What ports are you visiting? You can always google "WEather XX December" and get the average temperature and rainfall. Weather will be variable, and you won't be needing to pack your swimsuit.
I'd plan for generally cold, rainy and windy (which means rough seas). If you're prepared for that, it's likely to be a little better for at least part of your trip (all if you're lucky).