its a bit confusing but i think the water bearer represents the bearer of knowlege and the waves represent the waves of knowlage and air signs are known to be the most logical and smart of the zodiac.
Also, adding on to what Princessofpower said, air as an element means that the zodiac is "light" and free. They value their freedom, and cannot भालू to be tied down. So, Aquarius is air because they prefer freedom and travel supposed to earth, stability(especially in relationships) and rules.
