2011 Buick LaCrosse - Water on floor

water leak on rear passenger floor

this video is for a 2010 but yours should be the same. seems it happened to others that the drain comes out. there might be a drain hose on both sides ot the trunk so check for it.

2010 Buick Lacrosse sunroof drain - YouTube

Another possibility is the rear door drains are clogged. Windows that open & close do not seal 100%, so rain water always gets past the seal & inside the door. No worries, inside of door is water resistance, and any water that gets in has a designed-in pathway to just drain out the bottom of the door and outside. If drain holes are clogged, water may drain into car instead. Open rear door, use a mirror to inspect door underside. You should see a bevy of small drains holes. Make sure they are open.

The very brief post says water on rear passenger side .

They did not ask for help so this is just another of those complaint posts .