Tina,
My buddy purchased the 7D as well. He also had issues with "tack sharp". He lives in NJ near one of the Canon repair centers. He had the Canon tech reps look at the camera and lens combination that he uses. A 70-200 F2.... and a prime 400mm F 4.... I think. In any case Canon found that the sensor was not placed in the exact location it should be when the camera was made. I would find out the nearest location of a service center and bring or ship the camera to them. Call first so you get a authorization number and explain why you are returning the camera. By the way it took 3 trys for Canon to get it right.

What kind of waterfowl are you photographing?

Ducks floating in a backyard pond, or cormorants and anhingas fishing and flying around a marsh?

For action shots you should try to keep your shutter speed up as high as possible. Fast action=over 1000.
In addition to stopping the action, a fast shutter speed will cut down the possibility of soft images due to camera shake and movement while panning.
If the birds are not super close DOF is not a huge issue.
5.6, 6.3 is fine.
Select an ISO that will give you a similar lens opening at the shutter speed required for the lighting and subject matter.

Birds just sitting or floating do not require a super fast shutter speed, just fast enough to prevent lens/camera shake.
Birds busting up out of the water with a fish do require a fast shutter.

I use center point expanded on my 7D for birds and wildlife.

Also...you are walking around out there with over $3,000 worth of camera gear...you should learn how to spell Canon. :P :D