What is the definition of a "seaplane" according to the rules?

The definition of a "seaplane" according to the COLREGs refers to an aircraft specifically designed to operate on and maneuver in water. This capability distinguishes seaplanes from other types of aircraft that may only be intended for flight without the capacity to take off or land on water.

A seaplane is typically equipped with features such as floats or hulls that allow it to float on the water's surface, making it suitable for both takeoff and landing on water bodies. This dual functionality is essential for their classification within the maritime context, as it impacts navigation rules and protocols that pertain to vessels and aircraft.

The other choices do not accurately capture the complete function and purpose of a seaplane. An aircraft designed solely for flight lacks the water operation aspect, while a vessel operating only above water does not encompass the entire operational capability of a seaplane. Additionally, defining a seaplane as a watercraft used for fishing is misleading, as it does not reflect the primary design and operational characteristics of a seaplane.