Probably quite good for that use, once Ikelite et al. come out with a housing. Of course, the housing will likely be almost as much as the 6D, more including the lens port.Yep, this is an important thing to consider, if $2000 is your budget, the 6D won't work because to actually take it underwater, you'll need $4000+. This also presumes you have a lens that pairs nicely for that purpose.
The -3EV focusing could be really useful, but, beyond that, I'm not sure you'll see a big increase over the 5dII. What you may be able to do is find better deals on the 5dII and underwater housings, since they would be getting phased out.