The mission begins with you in a cave, so feel free to look around a bit before jumping into the water. Start swimming, following the lights under the water, being quick as you don’t have a lot of oxygen and can drown. When you get out of the water, grab the SMG and dive into the next pool to eventually reach a spot outside, where you’ll find a jet ski waiting.
Drive the jet ski down the river, avoiding the debris and stuff that’s already in the river, as well as the vehicles and other junk that falls from the sky. Note that there will also be vehicles sitting in the river that explode when you pass them, so be careful not to get too close. At the end, go onto land and go to the back of the building to get inside, grab your gear and finish the mission.