For this achievement is for being eaten by a shark. What you will need to do is venture out into the ocean by any form of transportation.
I recommend being anywhere away from land and using a boat for this as you can drive slowly enough to find a shark. A shark will appear on the mini maps as a red dot.
Jump over board into the water and dive under and just swim towards towards it. As soon as the shark kills you the achievement will pop.
If you're busy like me, you can just leave your account idle as you are out in the ocean and you will eventually be killed by a shark at some point.