I was about three months along on one of our cruises, way back in 1993. First, even though you won't be showing, bring along a letter from your doctor or midwife indicating your due date, and other information the cruise line typically requests. (You can find these in the FAQ, or let me know which line you're sailing and I'll post a link.)
I did have motion sickness problems while on that particular cruise. I had brought along sea bands -- the ones with the pressure point on the wrist -- and they didn't work too well. Our waiter suggested that I have apples and crackers, and they worked! I still have an apple these days when cruising in rough seas. They're free, easily found on board, and have no side effects.
I also made sure I ate well while on the cruise, but it wasn't a problem. Once I told our waiter I was expecting, he made sure I had a glass of skim milk brought to me as soon as I sat down to dinner.
The only issue I found was that some of my clothing was already starting to feel a little tight. So, it's time to go shopping!
Congratulations again, and please feel free to ask any more questions that come to mind. Although the cruise was 14 years ago, I still have my journals.