The ornamental design for a waterheater, as shown and described.
FIG. 1 is a front view of a water heater showing my new design;
FIG. 2 is a rear view of the water heater;
FIG. 3 is a right side view of the water heater;
FIG. 4 is a left side view of the water heater;
FIG. 5 is a top view of the water heater;
FIG. 6 is a bottom view of the water heater; and,
FIG. 7 is a top front side perspective view of the water heater.
The breaks in the connections are illustrative of portions of the device forming no part of the claimed design. The broken lines are illustrative of environmental structure forming no part of the claimed design.