The ornamental design for a hydrotherapy apparatus, as shown and described.
FIG. 1 is a perspective view of a hydrotherapy apparatus according to the present invention;
FIG. 2 is a top plan view of the hydrotherapy apparatus of FIG. 1;
FIG. 3 is a side elevation view of the hydrotherapy apparatus of FIG. 1; and,
FIG. 4 is a front elevation view of the hydrotherapy apparatus of FIG. 1.
The broken lines immediately adjacent to the shaded areas represent the bounds of the claimed design while all other broken lines are for environmental purposes only, the broken lines form no part of the claimed design.