I have never sailed out of Galveston, but looking at the map it seems like many other ports I've sailed out of in FL. Just a short(ish) channel to get to open water, that I'd expect to be generally smooth at least until you get to open water. Though as mentioned already, you never know what the conditions will be like until that time gets there.
I have sailed out of Mobile and NO, neither of which I found rocky at all. Unless rocky means something different to you than it does me. Are you saying you had a rocky ride down the Mississippi until you got to the Gulf?