What is Aqua?

Legal Definition
Water. This word is used in composition, as aquae ductus, &c. 2. It is a rule that water belongs to the land which it covers, when it is stationary: aqua cedit solo. But the owner of running water, or of a water course, cannot stop it the inferior inheritance having a right to the flow: aqua currit et debet currere, ut currere solebat.
-- Bouviers Law Dictionary
Legal Definition
In the civil and old English law. Water; sometimes a stream or water-course.
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary