If you want to kill sharks you have to shoot them, or blow them up. Snipe them from the beach, go out in a boat. Just try not to kill them in too deep water so you don't have to dive to skin them 'cos even if you don't drown one of his friends will probably nibble you on the way back up.
Use the yellow hunting syringe to see the sharks in the water by there silhouettes, then use a sniper rifle and shoot from a distance, takes two shots. Try to get the ones who are closer to shore so they don't sink to deep, plus your are farther from the other sharks.