For underwater, a separate wide angle and macro lens is the standard equipment. I personally use the 17-40 F4L and the 100mm macro on a 5D, which would be comparable to the 10-22 and 60mm macro on a smaller frame 7D. Typically, the general purpose zooms are not wide enough on the wide end nor do they focus close enough for good macro, if they have a macro setting, as scottfiji mentioned. My recommendation is that you get dedicated wide and/or macro lenses. I feel that a cheap general purpose zoom will disappoint compared to something like the G11 in a housing with wet removable lens.
As for Prime vs. zoom, topside the issues these days is a little mixed. Back 15+ years ago there was a clear advantage for prime lenses as they have the best image quality. These days top quality zooms (such as the canon L series), do very well and would be just as good aside from one caveat I'll get to. The basic/cheap zoom lenses typically will not yield the optical quality of a similarly priced prime lens.
Now, within the last two or three years, sensor resolutions have gotten so high that, once again, there are optical advantages to the best prime lenses over the best zoom lenses. However, unless you are making enlargements up to feet across and have thousands of dollars to spend, the best of the best primes aren't necessary.
Also, while underwater a zoom is often less ideal in a housing, especially if the length of the lens changes while you are zooming. In the worst case scenario, the lens will not fit in the housing or bad vinigretting at one end of the zoom or the other. The best case problem has to do with what is called the entrance pupil of the lens, but don't dwell on this one, most recommended lens ports account for this.
