There are 15 Whiskey Cask Locations in Far Cry 5. Finding all Whiskey Casks is needed for the Whiskey River Side Mission. After you accept that mission you can buy a map for all cask locations from any vendor in the game for $1200. Finding one of them is also required to unlock the Pack Rat Trophy / Achievement.
Whiskey Casks are only found in Jacob’s region, Whitetail Mountains. They are all sunken in the water. You need to dive down to locate them. The perk “Human Fish” lets you swim faster and hold breath for longer which makes this easier. It Only costs 1 perk point.
In the pictures below all the locations are circled in red color:
For all other FC5 Collectibles check out these guides: