Hey team,
The best way to get this is to fall into deep water from a 100m drop. Deep water is dark blue on the map.
Slight story spoiler:
*** Spoiler - click to reveal *** The main story Willis mission, when you fly to the second island, is a quick way to get it. When you jump out of the plane just fall down, yes the mission will fail however you will net 15GS.
If you already did that mission, or you don't want to open the spoiler, you can travel to X805 Y745 and jump on the glider. Ride that out to sea (pulling back on the left thumb stick so your nose doesn't dip). Wait until the water changes to a darker blue, jump off and free fall into the water.
Good luck.