Research Base - Tents Pitched on Frozen Pond

Head up the hill from the Weather Station Base Camp to find a campsite set up on a frozen pond. A low wall of branches prevents you from simply swimming across, but there's a hole in the ice near the second tent.

Swim under the water toward that hole. As you do, 2 enemies appear on the ice above.

Hold LT/Right Mouse to remain submerged as you watch the soldiers through the hole. One of them moves off, while the other crouches down to work on an ice-cutting machine.

Surface next to him and when you see the Melee icon above his head, press Y/F...

...to pull him into the water and drown him.

Stay quiet as you climb out of the water, since the second enemy is still nearby.

He's not wearing a helmet, so you can take him out with a silent headshot...

...or sneak up behind him and use a Melee attack.

There are more enemies in the area beyond the red truck, but before taking care of them, move around to the far side of the pond, beyond the rocket-looking thingy, which I assume is some sort of meteorological equipment...

...to find a strongbox (1/2) containing a FULL-AUTO SHOTGUN PIECE (3/4).