But I can not confirm that FF wideangles deliver poorer corners in general. I think we have been spoilt by FF lenses on cropped sensors where corners .
Do you mean topside or through a dome port, Julian? Pretty much all lenses, and all the practical underwater options, will be a bit soft in the corners on full frame when shot wide open in above water use. If you want really sharp corners on the 5D, you're pretty much talking Zeiss 21, Zuiko 21 and Leica Elmarit 19mm - but I don't think you want to take any of these diving. In reality, the corner performance of something like the 17-40L would be fine for underwater use from the point of view of the lenses' optics.
I do wonder whether the softish corners (from the dome port, not the lens) that I see when shooting underwater on the 10D would be significnatly worse if shooting full frame. Even if Ikelite for example decides to house the 5D, I'm figuring that there's no experience available in using a port system like this for full frame. I guess I'm thinking that all the issues like nodal point etc would be magnified if we're using the full lens and not just the centre crop.