Why seaplanes carry vertical plates above their wings

What are the vertical plates above a seaplane's wings for?

They improve roll stability and clean up airflow, making water takeoffs and landings smoother as the aircraft moves between two very different surfaces.

The plates are aerodynamic aids rather than armour or simple structural braces. They improve roll stability and clean up the airflow over the wing. That control matters on a seaplane because the transition between water and air introduces changing friction and uneven loads. A later follow-up links similar devices on other aircraft to the same stability and airflow-management problem.