I claim the ornamental design for a submarine, as shown and described.
FIG. 1 is a perspective view of a submarine;
FIG. 2 is a second perspective view of the submarine shown in FIG. 1;
FIG. 3 is a bottom view of the submarine shown in FIG. 1;
FIG. 4 is a top view of the submarine shown in FIG. 1;
FIG. 5 is a left side view of the submarine shown in FIG. 1;
FIG. 6 is a right side view of the submarine shown in FIG. 1;
FIG. 7 is a front view of the submarine shown in FIG. 1; and,
FIG. 8 is a rear view of the submarine shown in FIG. 1. The broken lines of FIG. 8 are included for the purpose of illustrating a portion of the rear view that forms no part of the claimed design.