The ornamental design for pair of otter swim fins, as shown and described.
FIG. 1 is a perspective view of my pair of otter swim fins design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a sectional view taken along the center of the top view thereof; and,
FIG. 8 is a bottom view thereof.
The article shown is the design for a left foot swim fin. The design also includes the article for the right foot which is not shown but is a mirror image of the article shown.