There are a lot of -mancies -- which is to say, methods of divination. Yes, even necromancy: that was, originally, the summoning of the dead to learn the future from them -- it was only later that it came to mean just the summoning of the dead regardless of purpose and the suffix reinterpreted to mean a type of magic. It's safe to say that pretty much any stochastic process that exists in nature has been used to a) foretell the future, b) bet upon, or, most commonly, c) both. In this case, the random events are, when using stones, the sound of the dropping and the shape of the ripples, which then get interpreted by the divinator (much like tea leaves, there is much flexibility in how this is done); and when using oil, a little is poured on the water and the shapes interpreted. How scrying in water, or hydromancy, is different from this, I don't understand. The word comes from Greek lekanomanteía, from the roots lecano-, basin/bowl + manteía, divination -- it was known to have been practiced in Mesopotamia, and it's possible that it's what Joseph is doing in Genesis 44:5.
Have you a mind (quoth Her Trippa) to have the truth of the matter yet more fully and amply disclosed unto you by Leconomancy, of old in prime request amongst the Assyrians, and thoroughly tried by Hermolaus Barbarus.
—Thomas Urquhart, translating Rabelais, Gargantua & Pantagruel, iii. xxv