The ornamental design for a stand-up paddleboard, as shown and described.
FIG. 1 is a perspective, bottom view of a stand-up paddleboard showing my new design;
FIG. 2 is a bottom view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a cross-sectional taken toward the front of the design thereof; and,
FIG. 7 is a cross-sectional taken toward the back of the design thereof.
The showing of evenly-spaced broken lines is included for the purpose of illustrating environmental structure and forms no part of the claimed design. The dot-dash boundary line demonstrates the boundary of the claimed area.