Police boat

A police boat is a small watercraft used by the Metalbending Police Force for water operations

History
They were first seen when Chief Lin Beifong had the Metalbending Police Force provide security for the Pro-bending finals because of a threat by Amon of "severe consequences" if the arena wasn't closed and the finals canceled. The boats were stationed in the waters all around the Pro-bending arena. However, after Amon was making his escape from the arena, the waters around the arena were lit up with the fiery wreckage of the police boats, implying their destruction.

Design
They appear to be powered by gasoline, since there is no smokestack. They are small, perfect for patrolling the waters of Yue Bay