In the Japanese mission Attack on Sydney Harbor, there is an underwater cave tucked into a corner. Proceed with the mission as normal until you are past the fourth waypoint. A checkpoint will trigger and you will no longer set off an alarm if you surface. Open your tactical map and locate the river that goes north and west above the main harbor inlet. The cave is at the end of the dark blue (deep) water, and you will need to be heading west to enter it. If you zoom the map in, you can actually see that instead of the gradual change to light blue and then sand and then grass/foliage, the water has a distinct cutoff with a darker brown colored terrain. This is essentially a hill that has the hideout underneath it. When you enter the cave the achievement will unlock.