A two-piece cap for a water bottle comprises a main cap which has an annular top disk formed with a depending central well having three levels, each of a slightly smaller diameter. The main cap skirt which depends from the periphery of the top disk has an upper cylindrical portion formed with a truncated internal screw thread which merges into a horizontal bead extending about 180.degree. around the interior of the cap and terminates in a reverse thread portion which extends upward to merge with the leading edge of the truncated screw thread. Below the upper skirt portion is a downward-outward slanted conical portion merging with a lower cylindrical skirt portion having an internal locking bead. A first vertical score line extends from the bottom edge of lower skirt portion to the upper edge of the slanted portion. A second vertical score line, spaced a short distance from the first vertical score line, extends from the bottom edge of the lower skirt portion to the top disk. A tear tab depends between the two vertical score lines. The main cap fits onto a bottle neck having a top portion externally threaded to mate with the internal thread of the cap. Below the top bottle portion is an external bead under which the internal locking bead of the cap seats to make the cap tamper-evidencing. A plug having a closed bottom and a smooth exterior fits into the bottom of the well. The plug has an upper taper to assist in inserting the plug into the bottom of the well followed by three levels, each of slightly larger diameter to seal the central well. An external bead limits inward movement of the plug into the well. An internal bead in the upper end of the plug seats in a groove adjacent the upper end of a water dispenser tube.