The ornamental design for a waterbottlecap, as shown and described.
FIG. 1 is a front view of a water bottle cap.
FIG. 2 is a left side view of the water bottle cap.
FIG. 3 is a right side view of the water bottle cap.
FIG. 4 is a top view of the water bottle cap.
FIG. 5 is a rear view of the water bottle cap.
FIG. 6 is a front left isometric view of the water bottle cap; and,
FIG. 7 is a front-right isometric view of the water bottle cap.
The broken lines showing are included for the purpose of illustrating the environment of the design only and form no part of the claimed design. The stippling in the drawings indicates a contrast in appearance.