I'm just starting to use the OM-D for video underwater. Here's what i'm doing:
Using continuous autofocus (C-AF) works very well. C-AF+TR should be very useful in one case (keeping focus on large pelagics where the background is open water). But if you are shooting a subject against a wall, reef, bottom, kelp, etc. C-AF+TR will track your subject for a while then get confused and start tracking on the reef/kelp etc, and/or then just loose tracking altogether... (Menu -> Custom Menu -> A. AF/MF -> AF Mode -> Movie -> C-AF.) Note that half-pressing the shutter while shooting video will re-focus, but there's generally an objectionable amount of focus hunting, so S-AF and half-presses of the shutter are a really bad idea in my opinion.
Assigning Fn1 to x2 conversion allows you to zoom in (2x) for video w/o any loss in quality (great for wide-angle lenses when you can't get close enough).
(Menu -> Custom Menu -> B. Button/Dial -> Button Function -> Fn1 -> Digital Tele-converter.)
It's a very good idea to learn to adjust the curves when shooting video. The sensor has a lot of dynamic range, but when you are shooting video you can't shoot raw and adjust levels later on, you need to adjust the curves before you start shooting. Hugely important if you are shooting available light in caverns, if you're shooting macro video with lights, then adjusting the curves is less likely to be necessary.
