Here is my walkthrough of the campaign, part 3, that shows exactly where to get this achievement.
1. This is done after diving module is attached, and after progressing through the underwater caves.
2. You'll be stopped by a big door.
3. You have to attach the crane on the back of your ship to a large door in the water, and pull it open.
4. Swim down and in, to the left, down a floor, and to the right.
5. You will hit a room with a heavy object. Open the roof, swim out, attach the crane to it, and drag it up to the boat.
6. While it's lifting up to the boat, dive down to where there's a hole in the floor just below where the item was sitting, and press the blue button to open the sea door.