The ornamental design for waterbed mattress, substantially as shown and described.
FIG. 1 is a front, top and left side perspective view of a waterbed
mattress showing my new design;
FIG. 2 is a side elevational view;
FIG. 3 is a front elevational view;
FIG. 4 is a bottom perspective view.
The broken lines inside the accented corners indicate slight changes in
contour and the randomly placed non-parallel lines on the rest of the
mattress are meant to show that the mattress is baggy.