The ornamental design for a pulley for a waterrower, as shown and described.
FIG. 1 is a right perspective view of a pulley for a water rower showing the new design.
FIG. 2 is a rear view thereof.
FIG. 3 is a front view thereof.
FIG. 4 is a left side view thereof; and,
FIG. 5 is a right side view thereof.
Any shading and cross-hatching are not features of the design but are utilized to illustrate the surface contours of the pulley for a water rower design in the drawings. It will be understood that the dashed lines presented in the drawings are for illustration only, and do not form a part of the claimed design.