I know Aquarius is supposed to be an Air sign. However, it is a jewelled vase pouring Water. And its name has "aqua" in it. Even the Water Halfblood's name is Aquarius. So personally, if I were you, Zanz, I'd make Aquarius a Water zodiac.
By the way, this is my Element-to-Zodiac alignment:
Aries: Light
Taurus: Gravity
Gemini: Aether
Cancer: Darkness
Leo: Fire
Virgo: Life
Libra: Time
Scorpio: Death
Sagittarius: Air
Capricorn: Earth
Aquarius: Water
Pisces: Entropy
My own set of triplicity:
Cardinal: Time, Aether, Gravity, Entropy. Those elements form the laws governing "space". They define time, space, order, and chaos.
Material: Fire, Water, Air, Earth. Those elements form the basis of "matter". They define hot, cold, solid, and intangible.
Spiritual: Life, Death, Light, Dark. Those elements control the lives of "creatures". They define growth, decay, good, and evil.
My triplicity was explained in my story in the Short Story contest.