I don't think there can be any guaranteed shark-free zone in salt water, just as there are no lightning-free zones on land. When your time is up, its up! Take comfort in knowing your name will be in the local papers!
I think the real element here is a millenia old fear humans have of being eaten whether it be shark, lion, boa, canibals. Few methods of death raise such anxiety; except for maybe fire and drowning. (At least we avoid one of those by diving!
My personal experience with dangerous sharks is limited to the GW at Molokini (discussed here before). I have over 600 dives, mostly in So Cal and have never seen a dangerous shark in person locally...but they are here. I had to go to Hawaii to see a shark that should not have been there...go figure.
I do admit I get a bit nervous entering a murky surf zone on the local beaches...especially when I am the only one in the water. The mind plays tricks...every free floating piece of seaweed is a potential killer!!! The occasional ambush by my harbor seal friend can stop my heart.
My real fear is the plane crashing on the way
TO the dive destination.
Bottom line: Dive easy, your fate does not include "death-by-shark".
Jon