The easiest way to fight sharks while underwater is to avoid fighting sharks while underwater
Whenever you plan to do some deep sea diving, send out Ikaros first to spot any sharks you might run into. Then, just pick them off from the surface with your physically unrealistic bow. I've had this work with sharks of any depth.
If you find yourself face to face with a shark, maximize your range
Make sure you have a quick weapon with length (like a spear or staff) that can hit/stun a shark before it makes contact with you. That's usually enough to stave off an attack and force them to circle for another pass at you.
Also, if you find yourself surrounded, quickly changing your vertical position (moving up or down) is a more effective way of dodging then just moving horizontally.