I shoot with a pair of 1.6x Canon DSLR cameras (presently my cameras are the 7D although I have traveled through China with a 30D and 40D). I am quite happy with two lenses, 17-55mm f/2.8 IS and 70-200mm f/4L IS. Many folks don't use longer focal lengths for travel but, I would not travel without a longer lens for isolating people and compressing distances.
Longer focal length for people...
173mm (276.8 equivalent)
100mm (160mm equivalent)
138mm (220.8 mm equivalent)
Long focal lengths compress distances..
70mm (112 mm equivalent)
85mm (136mm equivalent)
127mm (203.2mm equivalent)
I do, however often throw the older 28-135mm f/3.5-5.6 IS lens in my kit as a redundancy or backup lens which "could" if needed fill in for either my 17-55mm or 70-200mm lens if either goes down. The image quality is reasonable and the lens is fairly light weight. The lens will stay in my hotel room.
I can shoot at ISO 640 or so with quite reasonable quality. This allows me to capture available light images using the 7D and 17-55mm f/2.8 IS lens.
1/400 @ f/5.6
1/125 @ f/5.6
1/125 @ f/5.6
One accessory that I would bring on any cruise is a CPL filter...
I will also carry a small hotshoe flash, the Canon 270EXii which I will bounce with a Demb Photojournalist FlipIt. This gives me some versatility indoors...