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