The easiest point to die to sharks is at the raft with the GPS locator on the west side of the island. Swim out there and right in front of the shark's face. - (Confirmed By Plasm)
There Is Also A Guaranteed Shark In The Rebreather Cave, Which Is Right Next To The Beach Helicopter Crash Spawn Point. (Northern-Most Big Beach)
- Walk In, And Hug The Left Hand Wall Until You Get To A Large Cave-Pool; The Shark's In There.
- 4 Or 5 Bites From Full Health (With No Armor) Gets You The Achievement.
- Swim In Front Of It's Face To Make It Bite You.
Good Luck!