The ornamental design for a swim fin, as shown and described.
1. Swim fin
1.1 : Perspective
1.2 : Top
1.3 : Bottom
1.4 : Left
1.5 : Right
1.6 : Back
1.7 : Front
FIG. 1.1 is a perspective elevational view of swim fins showing my new design; FIG. 1.2 is a top plan view thereof; FIG. 1.3 is a bottom plan view thereof; FIG. 1.4 is a left side view thereof; FIG. 1.5 is a right side view thereof; FIG. 1.6 is a rear elevational view thereof; FIG. 1.7 is a front elevational view thereof; Where included, hair lines which shows changes of curvatures are for illustrative purposes only and form no part of the claimed design.