The ornamental design for a multi-piece surfboard, as shown and described.
FIG. 1 is a top perspective view of a multi-piece surfboard showing my new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a right side elevational plan view thereof.
The broken lines showing the multi-piece surfboard in FIGS. 1-7 are included for the purpose of showing environmental structure and form no part of the claimed design.