The sharks can actually easily be killed, with a little skill.
Head towards them at full speed, machette or turkish knife in hand, and the second you see them turn for you, dive to the bottom, and turn in wide circles, never quit swimming.
In just a matter of seconds, they will stop chasing you, at which time, you swim almost past them, get under them, and start hacking with the right mouse button. You have to keep it up for quite a while, but when you succede, they will start to belly up, and you know you can head for air.