If you're unhappy with the results then consider sending the lens back to Canon to get it checked out. If it's new then it might be covered under warranty, and if it's old/used then it might be due for a good cleaning and re-calibration anyways. There's always a possibility that you just got a bad copy of the lens.
If you're looking at test charts like the link below the 70-200 f2.8 will show some softness wide open compared to the f4 L IS. It probably doesn't matter in the real world because a shot at f2.8 is impossible obviously with an f4 lens. The real question is whether it's worth the extra $ and the extra weight for the f2.8 if your main focus is shooting wakeboarding.
we were talking about the 70-200 (non is) ? right ?
would really suggest what you using it for and what your budget is..
i went with the 24-70 f2.8 and the 70-200 f4IS and use the 24-70 for my 2.8 shooting (that lens is was on my camera 80% of the time).
Both are super sharp lens wide open. the 70-200 f4IS and non f4IS are great zooms for wakeboarding shots. I seriously doubt if you need the 2.8 (IS or non IS ) unless you want to use it for something else.
please keep in mind i am a newbie and have used the 70-200 f4 (IS and non IS) and the 70-200 f2.8 (non IS).