We live in Texas so we've cruised out of Galveston a good number of times. The shelf in the northern gulf of Mexico is what gets most people who experience motion sickness and I, unfortunately, am one of them. There are deep currents out there due to deep slopes and there are good sized ocean swells... it is not pleasant.
I would say that the likelihood (potential roughness as you put it) is high. I've not once gone out of Galveston and not felt the motion of the ocean and retreated to my room on my first night. And, unfortunately, it doesn't matter where on the ship my room is.
If you're prone to motion sickness the gulf waters the month of December is not fun. That's not to say that I won't continue to do cruise out of Galveston in December, but I know what to expect and how to attempt to manage it. The good thing is that it is short lived and the whole trip does not feel this way.