The mission starts wen you pick up the bottle at Rockview Rest Stop. The bottle is on the platform that hands over the 'main' part of the oasis, where you'll be driving a sand skif around.
The bottle is hidden behind a board fastened to the inside of the ship's hull. When in the ship, use the exclamation mark on your map for guidance.
Enter Wurmwater and hang a right, into a large angry camp. When you can move more 'in' to the map, you'll see palm trees overhanding from the cliff. This bottle is sat upon one of those.
The bottle is located on a ledge beside the stalker-infested tunnel behind Spire Town Docks.
After killing The Big Sleep and The Sandman, drop onto the 'ledge' coming down from the waterfall. This bottle is sat here.