Focussing can be helped in two ways:
1) Use an external monitor...no different to a normal video system.
2) Pre-focus each scene using the "AF On" button at the back of the camera...no need for mechanical focus gearing. This is actually a better way to shoot video UW ...AF in my Sony HDV was always "hunting" in open water scenes, so I eventually used manual focus more often.
Housing design will be crucial in terms of ergonomic access to video controls...so far most housings (except the Aquatica) will require custom mods to make them useable for serious UW video. Also, I agree...unless necessary, stick to either video or stills per dive.
The more I use the 5D2 the more impressed I am with how it lends itself for the way I need to shoot video...in a more controlled and disciplined way. And the resultant HD video quality is far better than anything less than a RED...IMHO. With a good (cheap) Follow Focus rig and a viewing system (a la Red Rock, or clone) , the 5D2 is a lovely video camera to work with on land...even hand held.
By the way, yes Canon are working on a "Real" video camera...smaller cmos chip size, will probably take EF lenses, but will have dedicated lenses with special AF motors for video work. ETA...2010 or 2011. Might even be 4k capable...estimated cost around US$10k...so, yes it will be a very different animal aimed, no doubt, at the RED/Scarlet crowd.