I believe Canon has a software utility that will display the focus points. They should also be visible on your camera's rear LCD. In my experience, shutter speed is one of the main reasons for poor pictures; either too slow to properly freeze subject movement, or too slow for the focal length of the lens being used.
Yep. The Canon DPP software included with your camera will show the AF point used to auto focus.
If you have a UV or Clear filter on the front of the lens for "protection" take the filter off.
What focus mode, focus area mode, and focus point(s) were you using?
You might want to review pages 85 thru 140 of your Canon 7D Mark II Instruction Manual.
Was the camera hand held or was it on a tripod?
Was OS turned on on the lens?
Which Sigma 70-200 lens?
Canon 7D Mark II, or 7D II - it took me a second to understand what 7Dii meant.
The II is the roman numeral for 2.
FWIW - the less than good quality light in the scene - bright background, shaded foreground - is part of the 'not clear' issue.
The aperture, f5.6, defines the amount of light being let into the camera by the lens and also affects the depth of field. For this kind of shot f5.6 is a solid choice, you might use f4 if you wanted a little less depth, even down to f3.2 I think would have been fine at this range; unless you wanted it all in focus (bird and water and background) then you'd want to use a smaller aperture (bigger f number).
ISO 200 isn't bad, a nice clean ISO
1/100sec - here is your problem in two fronts.
a) Handshake - the rule of thumb is 1/focal length of lens for the minimum shutter speed for a handheld shot to be sharp. That's a rough guideline and sometimes you'll find you have to use a faster shutter speed and sometimes you can get away with slower. So at least you want to be at 1/200sec at the 200mm end of your lens.
This type of blur appears as an even level of blur over the whole photo.
b) Motion blur of the subject - for a moving subject you've got to have a faster shutter speed to stop it blurring. 1/100sec is really too slow and risking it; for wildlife I'd say get up to 1/500sec if you can or faster.
This type of blur appears as blur around moving parts of the photo only
Note you can get, of course, both motion and handshake blur at the same time in a photo.
Thus based on the above you want to use a faster shutter speed so you've three options:
1) Use a wider aperture (smaller f number). Affects your depth of field; but also lets in more light and thus allows you to use a faster shutter speed
2) Raise the ISO. Yes a higher ISO means more noise; but well exposed photos at higher ISO are fine! You can fix noise in editing and pritning/display typically hides a lot too; but you can't fix motion blur*. So raising the ISO is important. For wildlife be prepared to use ISO 1600, 3200 or even higher!
3) A combination of a wider aperture and a higher ISO value.
*Practically speaking that is; in theory you can fix it but it will take so long its not worth it in most cases