After all the combinations I tried, the best for me is just the center point and tracking sensitivity is set to moderate slow. Unlike 50D or 5DII, 7D\'s center point is very potent. I was carried away with AF expansion, while it was joy to track with this set up, perfect focus shots were very few compared with that of center point only. Again, center AF point only gives the best compromise specially with busy BG, and I bump the back AF botton to acquire focus if needed(to overcome the slow tracking sensitivity).
All shots with 400L at about 400-800 ISO.
Note that fast flying ducks against to water BG is not so easy like blue sky.
Full frame:
