The ornamental design for a dolphin/shark hook fin on a swim vest, as shown and described.
FIG. 1 is a right side view of a dolphin/shark hook fin on a swim vest, according to an embodiment, the left side view being a mirror image of the right side view;
FIG. 2 is a back view of the dolphin/shark hook fin on a swim vest illustrated in FIG. 1;
FIG. 3 is a top view of the dolphin/shark hook fin on a swim vest illustrated in FIG. 1; and,
FIG. 4 is a bottom view of the dolphin/shark hook fin on a swim vest illustrated in FIG. 1.
The thin solid lines illustrate the contour of the claimed design and are not a claimed surface pattern. The broken lines are directed to unclaimed portions of the design and form no part of the claimed design.