The 'most-used' lenses on FF Canon for underwater use (for people shots) are the 17~40 and 16~35 zooms and then fixed focals in the 24~35mm range (Canon's 20 is not a very good lens underwater IMHO, the Sigma appears rated as better but I've not personally tried one)). As Tom says, its also very much about the dome port so you need to make sure that Ikelite build a dome set-up suitable for the lens you choose OR check what domes they produce and buy a lens accordingly. The set-up should optically align lens and port too.
My own favourite lens on the 5D is the 24/1.4L (which is expensive) and I generally use it with a +3 diopter behind a medium dome. It gives a very bright fast focus lens underwater but may be a little wide depending on what you are doing. You could look at the 24/2.8 and 35/2 as a cheap combination which will be usable behind a similar dome but again take note of Tom's comment not to use wide apertures or the corners will be soft. A Tom also says, do have a look at
www.wetpixel.com for masses of information about underwater photography.
