We claim the ornamental design for floating platform substructure, as shown and described.
FIG. 1 is a front/top/right-side perspective view of a floating platform substructure showing our new design, the rear/top/left-side perspective view being a mirror image and, therefore, not shown; and,
FIG. 2 is a front/bottom/right-side perspective view thereof, the rear/bottom/left-side perspective view being a mirror image and, therefore, not shown.