"Find the missing McHelen's whiskey barrels submerged in the lake and bring them to Sherri. Remember: look for the red cross that marks the spot, and then follow the direction written near it."
As it says, check for the red cross in the area. Then for a read buoy, which is either on the surface or under water. There's a rope attached to the buoy that leads to the barrel. To retrieve the barrel you sometimes have to dive into pipes, collapsed structure / boats, caves or train wagons...