The ornamental design for a swimwear apparatus, as shown and described.
FIG. 1 is a front view of a swimwear apparatus comprising my design;
FIG. 2 is a rear view of a swimwear apparatus comprising my design;
FIG. 3 is a right side view of a swimwear apparatus comprising my design; and,
FIG. 4 is a left side view of a swimwear apparatus comprising my design.
The broken lines in the drawings form no part of the claimed design. In addition, the claimed design is not limited to any specific color, texture or markings, graphic or surface treatments.