To obtain the Red Tide side quest, speak with Carmen at the Santa Monica Pier in the Lifeguard HQ during the Rage Quit story quest. For this quest, you’ll need to pick up Cooper’s trail at the CDC HQ. Make your way all the way to the southeastern side of the area and enter the white tent.
Once inside, loot the Cashing Out journal from a yellow medical waste container near the door leading deeper into the tent and the Paging Doctor Whom journal from the table in the same room by the computer monitors. Next, head down the hallway to find another journal, Change of Scenery, behind some room dividers. Exit the tent and head northwest to the next white tent.
Interact with the keycard reader to enter then clear out all the Butchers! Make sure to pick up the Huang’s Warning journal that drops from one of the Butchers. Next, head to the beach in search of the boat. First, inspect the partially sunken boat in the water.
Thankfully this was not the boat we’re looking for! Unfortunately, you’ll find it under the southern end of the Pier. Once you arrive, you’ll need to search the red containers scattered around the boat. The one you’re looking for can be found south of the boat.
Be mindful that you’ll also have to clear out all the zombies in the area. Once you touch the red containers, more will spawn. When you’ve found the correct red container, Cooper spawns. Take out the mutating zombie to complete the quest!