I claim the ornamental design for the water cooler design, as shown and described in the drawings.
FIG. 1 is a top, front and right side perspective view of a water cooler design showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawing views represent unclaimed portions of a water cooler design, which are included for the purpose of illustrating unclaimed environment only, and forms no part of the claimed design.