"Push an overseer into the water" is an optional mission objective in Laying The First Brick in Freedom Cry
Walkthrough[edit]
When you make it into the Maroon Hideout, this objective will become active. Head over to the left and swing to the lower area. Make your way Stealthily over to the ladder and climb up it. An overseer will walk right up to the ledge. Perform a ledge assassination on him to complete this optional objective.