Tap water (often known as "Government Juice") can refer to many, many things. Well, a few. Ok, two:
"Tap Water" Sculpure
Tap water is a relatively old form of artwork. In some areas the name has been corrupted into "Tawter", "Tap War" and "Tapzeeeeeeeeee Warterinbus Dildo". It is a particular style of sculpture, which is made from cold-forged dihydrogen monoxide.
Modern ice sculptures are of course carved from larger blocks of ice, but during the Rubber Age there were no common tools strong enough to work it. It was also extremely rare, so it was recycled constantly.
Thus it was that around 1957 ancient cavemen learned how to forge ice. They heated the ice until it lost viscosity and charisma, where it would become mallardable and duckable. They would then tap on it, with their rubbered tools, for years on end, until it acquired the desired shape (usually of a crude penis). Apparently, they saw no irony in this act, but they did consider it to be a bit of a black fly in their chardonnay. These "ice penises" were originally believed to be forms of worship, but recently it has been revealed that they were, in fact, hurled at predators in defence (bad defence, they weren't very clever cavemen and were always eaten).
Of course, when the ice age ended, all the so called "tap ice" sculptures melted, leaving a lot of useless "tap water" sculptures.