I too have done some underwater work. If you truly want Nemo quality, than you'll need a team of programmers and a couple of years to get it just right. However, if quick and dirty works for you like these shots
than all you need to do is create a spot light with fog, add some 3D fractals to the fog to give it some particluates, and use an ocean shader as a gel on the light. As for distance fog, if the camera is locked like my shots, you don't really need it. You can animate your objects transparency in post to simulate coming out of fog, and blur the top edge of the alpha of your ground plane to make it look like it's falling into the fog. Thrown in some bubbles with a particle system, and you're good to go. I also added some warping and noise in post for good measure.
Hope that helps.