I am a large woman. Not once did I feel self-concious during our cruise. But then, I also go to the local pool and romp in the water with my children, no matter what my size. I think that if you are trying to hide yourself, you will draw more attention than if you just jump in and enjoy the moment!
Also, on our 4-day, I actually lost weight, even though I felt like we ate constantly. We also walked everywhere (helped that our room was at the front, and the restaurants and pools were at the back, and that the stairs were faster than the elevators!), we left the boat when it was docked and explored the islands, in fact, the only time we really rested was when we went to bed!
My husband, well, he did gain a few, but then he was bound and determined to get his money's worth of Mickey Bars, plus he didn't have to keep going back to the room to get this or that for the kids!
You will have a great time, do not fret over how you look. You will be smiling and happy in your pictures, no one will look at anything else but the joy emanating from you!