I claim the ornamental design for a decorative water fountain apparatus, as shown and described.
FIG. 1 of the drawings is a perspective view showing the front, top, and left side of a decorative water fountain apparatus embodying the design of my invention, the broken lines depicting a flower on top and tiki-style torches on the sides as typical environmental objects used with the decorative water fountain apparatus that are not part of my design;
FIG. 2 is an elevation view of the right side of the apparatus;