Eels are classified as fish, specifically within the order Anguilliformes. They possess many of the characteristics that define fish, such as gills for breathing underwater and a streamlined body shape that helps them navigate through water.
Amphibians, on the other hand, are a separate class of animals that typically undergo a metamorphosis from a juvenile form with gills to an adult form that can breathe air with lungs, such as frogs and salamanders. Eels do not undergo this type of metamorphosis; rather, they remain aquatic throughout their life cycle and retain their gills.
While eels do exhibit some unique features, such as their elongated bodies and ability to live in both freshwater and saltwater, these adaptations still align them with the characteristics of fish. Thus, it is accurate to categorize eels as fish rather than amphibians.