Rescuers Go Under is a mission in Far Cry: New Dawn that is hard to reach. This is due to the quest's location - the place where you can start the mission is far away from the roads. The easiest way to reach it is to go to that place after completing To Love A Boar mission. There aren't any threats in the starting location. However, the nearby forest is roamed by groups of bandits.
You need Grappling Hook perk to complete this mission. More information on "How to unlock the Grappling Hook?" page.
You come across an abandoned car. The note is next to it. You learn about a stash inside a cave. To get there, you have to climb the cliff - this requires you to own the grappling hook.
At the top, you will find a large water body. To reach the cave with the stash, jump into the water and dive. Follow the linear corridors. Use the grappling hook to move forward.
One of the submerged corridors is the biggest obstacle. Ropes are too high to grab. To reach them, you have to go down to the ledge located below. After that, jump to the place where the car is. Push the car into the water - you can now proceed forward.
Follow the linear path to the stash. There, you will find resources, ammo and perk points.