I come from a professional diving background where I did a lot of work with various camera systems. I was part of a commercial diving team that provided underwater services to the US Navy under a government contract.
I can't believe no one has suggested the 20 mm 2.8. to go with the Ike housing. This is an excellent lens if you stop down to f-8 or f-11 on the 5 D. It has great contrast and color rendition and is very affordable.
As far as the Ike housing is concerned, just keep an eye on the lens port to make sure it doesn't come unseated prior to entering the water. Once leaving the surface, the lens port will seal more significantly due to water pressure.
Hope this helps.
Thanks, it does help. I hadn't considered the 20mm because of the f/2.8 maximum aperture, but it appears to be a capable lens. Thanks for the suggestion.