The standard zoom lens is the Canon EF-S 17-85mm f4-5.6 IS USM lens. I've found it works very reliably, the autofocus is very fast, and the zoom range is suitable for indoor use, small groups outdoors, and landscapes. My WHO - Show of Force photos were taken with this lens. I expect this lens will be my go-to lens for all the shore excursions on the trip.
This weekend in Stamford, CT, I shot these photos with an older version of the EF 70-300mm f/4-5.6 IS USM (the model I used is 75-300mm and somewhat older). This lens has excellent telephoto capabilities -- it compares favorably with the 400mm Sigma I used on my 35mm SLR many years ago. But the telephoto range is also a drawback: the wide end of its range is still pretty narrow. I can focus on an object no nearer than about 6' and at that I can see only 18" across in frame. The focus on this lens is external, which means that the lens extends about an inch when focusing on the near end of its range, and, the autofocus mechanism is noticeably slower than that in the 17-85mm lens. Consequently I expect I'll use this lens mostly while on the boat, shooting more distant objects. I expect most of the shore excursions will present interesting photographic targets more near than I could image with this lens.
I also plan to bring a EF 50mm f/1.8 II which I bought with the camera body. This lens is fast and serviceable for indoor, low-light use but I expect it will be as much my backup lens, should something happen to the others, as anything else.