Waterbending often appears to be one of the weaker bending styles, as it has a healing component, and many of its fighting techniques look more like a water balloon fight than a martial art. But that doesn't take into consideration how much water is in every living thing. According to the US Geological Survey, a human body is 60% water. Most of that water is in the brain, heart, and lungs. Even humans' bones are almost one-third H2O. The other bending techniques mostly throw stuff at opponents; waterbending can throw you at yourself.
The "A:TLA" episode "The Puppetmaster" introduces Hama, who invented bloodbending. Hama can control people by manipulating the water in their blood, making them dance like puppets on strings. Katara learns bloodbending from Hama, as well as how to extract the water out of plants. This process leaves any plant life in a certain radius completely desiccated.
It's a good thing Hama never put these skills together. In theory, a waterbender could suck the water out of a human, turning them into dessicated husks. They could also stop their blood from moving at all, killing them instantly. Waterbenders could even break every bone in someone's body from the inside out with some well-applied bending. A coldhearted waterbender could also flash-freeze their opponents by turning their blood into ice. Basically, the world of "Avatar" is very, very lucky that the most powerful waterbenders have also been kind.
