I wonder if just spraying CO2 in a shark's face might make it decide to go away? That is, if it's just being inquisitive, and not in full-on "attack mode." Curious sharks are a lot like toddlers. If they see something interesting, they put it in their mouth. That's the cause for most shark "attacks."
Next time you finish off a 2 liter bottle of pop (or soda if you have to call it that), try cutting a hole near the bottom, then sit down, put your mouth on the neck, and inhale as deep as you can.
A lungfull of concentrated CO2 will really knock you off your feet! If it makes the shark feel the same way, that may be enough to make it look for easier prey, without hurting it. Sort of like bear spray.
Of course, just like bear spray, you'd want something more effective for backup.
And I don't really know if CO2 bubbles would dissolve in water fast enough to affect the shark. I don't want to be the one to test it.