Far Cry 5 > Prepper Stash Locations and Walkthroughs
How to get the hint:
There is a guy at the river shore south from Dutch's Island.
There is a note at the south end of the bridge to the south (Bridge of Tears) of his location.
Stash Note Location Map
Stash Note Pickup
For the next bit you will need the GRAPPLE Perk from the ASSASSIN branch. From here you can use a zip line to get underneath the bridge. Here, use the Grapple to swing twice to the next wooden platform. DETACH to let go to land on it.
Cross the platform to the other side. There is another grapple location. Use it to swing 3 times to the next platform.
Climb over the big beam, go down one level. Here you'll see a big X on the opposite wall. Walk towards it, you'll break through the floor and you're at the stash.
Don't leave the 3 perk magazines
The floor you will fall through...
To get out, climb up where you broke down through the floor and exit the same way you came in. There you can simply jump into the river.