If you go 320 meters west of the Lazy Shore Marina on the South Island, there’s a shallow cavern built into the south face of the ravine. This Relic is inside.
You can’t drop down to the cavern from above, as the slope above it is too steep to avoid taking damage. You have to climb the hills north of the ravine and use your wingsuit and parachute to glide in, which may take a couple of tries to get just right.
This is further complicated by this entire area being guarded by some of Hoyt’s most aggressively paranoid troops, who will often attack on sight regardless of whether or not you have the privateer uniform. It’s a good idea to wait to go after this Relic until after the story campaign.