Prepper Stash: Salvage Rites
Find this stash at the Salvage Camp east of Clagett Bay in Jacob's Region.
The note is just in the middle of the campsite there.
You'll have to search the wreckage underwater. Each piece of wreckage is marked above with a buoy, and below with a green flare.
Some of these have loot, but the buoy leading to the key should be by an abandoned inflatable boat northeast of the camp. The flare lies by a cliff's edge, and the overturned nose of the plane.
It is possible to make these dives without the waterbreathing perk, but the margin for error isn't huge. When you have the key, return to camp and unlock the box for stash completion.