A water obelisk is used to create water orbs by casting Charge Water Orb which requires an unpowered orb in your inventory. It is located on a small island south of Catherby that is only reachable through a ladder in Taverley Dungeon.
The ladder to reach the obelisk is found in an area with black dragons in it, past the hellhounds and black demons. A player with 70 or 80 agility will reach this faster than running around the whole area, and doesn't require a dusty key to be used. Recommended you take an Anti-dragon shield with you in case a black dragon manages to breathe fire on you.
If you do any water orb, or even fire orb runs, a fast way to do runs is to leave through the one-way grappling shortcut and bank in Catherby, get your unpowered orbs, walk to the east, there where the fruit tree patch is, grapple your way up, run through Taverley Dungeon again, and proceed charging orbs.