Also known as scrying, specifically by water (which, unsurprisingly, is also called hydromancy). According to Wikipedia this can include the study of the ripples produced by pebbles dropped in a pool, along with some other more complicated methods involving drops of oil, rings shaken in bottles, or speaking words over a glass of water and studying its "spontaneous ebullience".
Hydromancy was apparently forbidden in Renaissance magic, along with geomancy, palmistry, divination by fire or weather, divination by bones - specifically shoulder blades (scapulimancy), and necromancy. It's not clear if divination by other parts of the body was acceptable, or why shoulder blades were so offensive. Demonology was also banned.