OK, you have the correct port extension, but you should still have some vignetting on the ff 5D MkIII. Anyway, as a start f4.5 is way inadequate, as eml58 says try f16 and I suspect your problems will be gone.
If not it will take some fiddling, put lens in manual focus and focus on the overwater section, take the housing out of the water dry off the port and take it off to see where you focus is, make a note, then do the same thing with the underwater part. You will end up with two figures, make sure you use dof/aperture value that comfortably encompasses both figures. Done.
If you look at the full res picture you can see that there is no vinjetting or at least as far as I can see
Might be a little darker in the corners but that's not bad.
I will try f16 and see if it will be better when I try to shoot with a smaller aperture.
Then I have to be at the correct distance to the object before I put it in the housing? Since I can't manually focus when it is in the dome.
Lets see if I understand it right.
I focus on the object that I want in focus ABOVE the water.. check the reading.. like 1m
Then I put it down in the water and to the same and check the reading.. and I get like 0.3m
I've attached a picture of the lens with the focus set on 0.5m (focused under the water).
How can I know the aperture/focusdistance to the "over"-object with this?