The Aquarius symbol (also called Aquarius glyph) is a shorthand representation of the Aquarius zodiac picture (see below). However, as with the picture itself, much is based both in interpretation and the preference of the writer, and can range from simple to extravagant.
The astrological symbol for Aquarius represents water (confusing as they're an air sign, yet strangely fitting as they always have to be different!) Above are some examples of commonly used styles for the Aquarius symbol. Each of these is equally valid - there is no 'correct' version as long as the general shape is maintained.
The word 'Zodiac' means circle of animals (though this sign isn't an animal in the normal sense - again, leave it to an Aquarian!..). These 'animals' were based originally on the pattern of the stars in their corresponding star constellation. Basically what the earliest astronomers and astrologers saw when they squinted at the night sky.
Due to this although Aquarius will always be represented by a water bearer, there isn't any one 'correct' shape or design for that water bearer. It's a cosmic game of join the dots, and some constellations even have alternative ways to form the animal from the dots (stars).
Any picture of the shape, head or profile of a water bearer can therefore be comfortably used here. Designs range from the space-age to the ancient, and are limited only by the imagination of the artist, as long as it gets the point across.
Below is a representation of Aquarius the water bearer (and the Aquarius symbol, inset) from a medieval wood carving from the 16th century. While it's not suggested this is a great picture (hey, you try carving something into the side of a tree which will look sexy in 400 years time!) it does illustrate that the basic concept of Aquarius the water bearer hasn't changed much over the centuries.
I have a separate article on Aquarius tattoos.