The symbol for water and waterbending is a circle comprising a stylized rightward-facing ocean wave with three spiral crests breaking over three wavy diagonal lines representing water.

The Avatar Symbols represent the Four Bending Nations and the elements they bend. They are fire, air, water and earth. Bending is the ability to manipulate an element.

Iroh drew the symbols of the bending arts in the dirt. The four elemental bending arts are based on the four classical elements, water, earth, fire, and air, each being manipulated through certain martial art styles that are reminiscent of the qualities of the element itself.

The world is traditionally divided into four nations: the Water Tribe, Earth Kingdom, Fire Nation, and Air Nomads. Each nation is culturally unique and associated with one bending technique: waterbending, earthbending, firebending, and airbending, respectively.

After some digging I found the water symbol on the waterbending scroll and the air symbol on Monk Gyatso's Amulet, but I haven't been able to find definitive sources for Earth and Fire.