The ornamental design for a swim top, as shown and described.
FIG. 1 is a front view of the swim top, showing the new design;
FIG. 2 is a rear view of the swim top;
FIG. 3 is a front perspective view of the swim top;
FIG. 4 is a front perspective view of the swim top as worn on the body; and,
FIG. 5 is a front view of the swim top as worn on the body.
The broken line showing in the drawings is included for the purpose of illustrating environmental elements of the swim top and forms no part of the claimed design.