Unmissable achievement as it's part of the story.
The bathroom scene has no bearing on gameplay.
You'll end up in the forest eventually and, after lighting a fire and burning a log, you'll be ambushed by scavengers and need to escape.
Thus follows a series of quick-time events using a mix of the arrow keys and point-and-click, typically to evade zombies and kick your pursuer away respectively.
Once you throw a rock, you'll fall in the river. Wait until you wash at the shore and the achievement is yours as soon as you regain control.