To free the straw hat prisoners in The Iron Hook in the Ghost of Tsushima jump across to the house and then immediately to the next house. Jump down the side of the house into the high grass and wait for the guard to come around and assassinate the guard.
Go across to the lookout tower when the guard is standing still infront of it and assassinate that guard and the lookout nearby.
Go back to the nearby cage and free the straw hat prisoners.
Go back to where you assassinated the lookout and wait for a guard to pass by and assassinate it too. Keep following the path to the cages, wait for the guard across the way to turn around and leave, and open the two nearby cages to free the remaining straw hat prisoners.