The ornamental design for a swim fin, as shown and described.
FIG. 1 is a right-side perspective view of the swim fin of the present design;
FIG. 2 is a top plan view of the swim fin of the design of FIG. 1;
FIG. 3 is a bottom plan view of the design of FIG. 1;
FIG. 4 is a right-side elevational view of the swim fin of the design of FIG. 1;
FIG. 5 is a left-side elevational view of the swim fin of the design of FIG. 1;
FIG. 6 is a front elevational view of the swim fin of the design of FIG. 1; and,
FIG. 7 is a rear elevational view of the swim fin of the design of FIG. 1.
The broken lines depicting portions of the bottom of the swim fin in the drawing are for illustrative purposes only and form no part of the claimed design.