I claim the ornamental design for the outdoor marking light, as shown and described.
FIG. 1 is a perspective view of the outdoor marking light of my new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof, the back side being a mirror image;
FIG. 4 is a right side view thereof, the left side being a mirror image;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines showing a dock, a cleat, and relevant hardware in FIG. 1 and showing a dock, a cap, and relevant hardware in FIG. 2 are for environmental purposes only and form no part of the claimed design.