A water tower is an elevated tank that is supported above the ground by several legs. Water towers are often used as intersection stations, or less commonly as references in a "to reach" description. Old water towers have often been replaced by newer ones in approximately the same location, so it is important to verify that the tower you find is the same one that was observed when the station was surveyed. You should verify that the location of the tower matches the location specified in the datasheet, and you should confirm that the features, such as the number of legs, are the same.
Usually a distinction was made between water towers and standpipes, so if the datasheet said water tower, but you find a standpipe at the location, it will usually mean that the original tower was replaced by the standpipe (or vice versa).