By the way, I don't understand why people seem to prefer shooting the OM-D via the viewfinder underwater.
The ability to use a large (tiltable!) screen, that is visible from a distance, to take pictures looks quite appealing to me, at least it's something I've envied to compact photographers quite often.
Can someone enlighten me?
I agree with you. I have a Nauticam Viewfinder ($1K) for a D7000 rig but I don't plan to use it on the OMD/Nauticam system. There may be occasional instances where bright light or glare may make bringing an eye close to the VF a necessity, for the vast majority of the time, having a compact system with Live View on the tilted LCD makes the most sense. If you want to see real focus, the magnify/teleconverter modes will help with this. The whole point of this system, at least IMHO, is that its a smaller lighter system. If I want to start adding a bunch of accessories, I may as use well the bigger bulkier D7000.
With regard to the post above, about increasing the autofocus speed, the setting to accomplish this is with the frame rate for the EVF, but its unclear to me whether this adjustment will increase focus speed irrespective of EVF vs OLED monitor, or if it only applies to the EVF mode.