The ornamental design for a waterflosser, as shown and described.
FIG. 1 is a top, front and right side perspective view of a water flosser showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is an enlarged top plan view thereof; and,
FIG. 7 is an enlarged bottom plan view thereof.
The long-short-long line of FIG. 7 defines the bounds of the claim. The evenly-incremented broken lines illustrate features of the article of manufacture that form no part of the claimed design.