There are large juice boxes. 1 at each gas station. You pick them up and carry them to the truck you are using. There should pop up when you get close for a spot to place them. When you have both you drive back to the lifeguard house and set them in the little spots in the corner across from the workbench.
NOTE: The gas station at the bottom of the map you need to climb on top and break through the glass ceiling on the back of it to get in.
there is a gas station down near the bottom right of the map that has them... you have to turn the power off which is in the giant hangar behind it. there are 2 thugs in there tho so be careful. But they are inside the gas station
You need to go to the gas stations and find two of the crates of juiceboxes. They will show up on the mini-map as quest items. The easiest way will be to drive to the gas station with a truck and put them in the back. When you get back to turn in the quest, there is a spot that you have to carry both crates of juice boxes to. It will also appear on your mini-map. Just carry them near that location and hit Square to place them in the proper location.