The ornamental design for a watertoy, as shown and described.
FIG. 1 is a front and bottom side perspective view of the water toy showing my new design;
FIG. 2 is a front elevational view thereof, the rear, left side and right side elevational views being identical to the front elevational view;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front and bottom side perspective view of the water toy in an extended state; and,
FIG. 6 is a front elevational view of the water toy in an extended state, the rear, left side and right side elevational views being identical to the front elevational view.