From the research station, head for the Cult Outpost. Halfway there, you'll come across a small shack with a green diamond icon. This is a Prepper Stash location.
Read the note out front of the shack to begin.
The boathouse is locked, but at least there are no hostiles.
To get inside, swim in & up from the water side.
The key is by the door, and there are a few things that be looted from the boathouse itself.
Throw the switch to restore power to the bunker.
Head back up to the bunker. The doors will open now, but it's still flooded.
Use the nearby pump to clear it.
Venture down and loot the place. Be sure to smash through the wooden barricade for even more stuff.
Looting this second room completes the mission.
This also where you can find the compound bow, your first stealthy distance weapon.