One thing to note is that behind a dome, the water/air interface creates a lens than produces a virtual image underwater. A 4.33" dome is going to mean max focus distances of only about 0.2 metres (for objects at infinity, less for closer objects) once you are below the surface.
I'm not sure if you've switched to using a single autofocus point on the E-M5. I haven't taken photos in such poor vis with the 8mm, but i have taken photos in very low light (shooting wide open at 1/20th, ISO 3200) and never had an issue with focus when using a single focus point. I just put the focus point on something with high contrast (diver's mask, hands, fish, coral, fins, etc.) where i want the focus. The super control panel (SCP), which you should have enabled, gives you quick access to setting the focus point just by pressing the OK button (and using the arrow keys and OK).
A (better) alternative to setting a single autofocus point, is to do it through the Magnify function. Assign a button to Magnify (i like to override the REC button since if i want to shoot video i can just set the mode dial to Video and press the shutter button). Tapping the Magnify button will bring up a single autofocus point, if you tap the button again it will magnify the image, i never use the actual magnification underwater, but when you do this you can spin the dial and set the magnification to 14x. This creates a smaller autofocus point. Tap the button a third time to go back to seeing the full image, but now with a small autofocus point. I like this because it gives me a smaller target for more accurate focus. Once you've set it to 14x, it'll always stay there and you'll always have the smallest focus point and you never have to enter magnify mode again (by pressing the button a second time.) To get out of the mode where you have a single small focus point, hold the Magnify button down for half a second to cancel.
Also i wouldn't use a focus light if there's any chance of it lighting up silt in the water -- that just creates an unwanted target for focus. The OM-D is very capable of focusing in low light once you take control of the focus point.
Edited by coroander, 05 November 2012 - 10:14 PM.