Can anyone suggest the best lens and underwater case for the 5D? I have a potential opportunity to go on a group dive and shoot some video and I need some help...

It's all depends on what you are gonna film... I'm a 7D user and I got a 8mm, 20mm and a 100mm macro for my Ikelite housing. So that's about a 14mm, 32mm and a 160mm. I use the 20mm for portraits on fish and close ups. For wide I use the 8 if it's not a lot of strait lines (on a wreck, it's a fish eye lens). I know that the 17-40L is popular around 5D users for UW.

Can anyone suggest the best lens and underwater case for the 5D? I have a potential opportunity to go on a group dive and shoot some video and I need some help...

The best lenses for UW with a full frame camera are 15mm, 20mm and 24mm.

If you only can use one lens and want to avoid lens changes, then use a good quality zoom. The best Canon zooms for UW are the 17-35mm L and 16-35mm L. The Canon 17-35mm L is my most used lens for UW filming and photography.