HI Tunc
looking at your picture of the diver next too the reef.
I'd have thought that all your dealing with is a DOF issue.
If your working at wide apertures even with your 12-24 you cant expect total front to back focus.
your diver seems to be a considerable way back from the reef scene.
Depending on your focusing point within this picture you would end up with different results. your divers face actually looks quite focused.
If you focused on your Diver, then i would anticipate that your foreground would be soft.
Depth of Field is always Less infront of your subject then behind it, Couple this with say F5.6 and even though its a 12-24 your going to get less than pin sharp results front to back
A good tip is try and get your diver in the same immediate area as your section of reef as near as you can get to your key focus area if you wish them to appear as sharp as your subject. and away from the edges as possible to avoid and distortion of their features.
Otherwise just use them in the backgound as a scene enhancer as Paul's cools example above
Take a look at the following image. Not a wonderful picture but you will see that the diver is very close to the my reef scene and will appear as in focus as the rest of the reef.
This was shot with a Nikkor 18-70 AF-S & +4 Dio i expect around F5.6 to F8. Lighting from 2x sb_105s Manual using Sea & Sea compact dome port.

Again the next image shot with the same combo. but the divers just set in the background.

Try and get some hard testing of your combo done using different aperture settings see how things come out.
regards
craig