We claim the ornamental design for a walk-in bathtub, as shown and described.
FIG. 1 is a front, top, right side perspective view of a walk-in bathtub showing our new design;
FIG. 2 is a front, top, right side perspective view with a door in an open position;
FIG. 3 is a rear, top, right side perspective view of the walk-in bathtub;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof;
FIG. 8 is a top view thereof; and,
FIG. 9 is a bottom view thereof.
The broken lines immediately adjacent to the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environmental structure; the broken lines form no part of the claimed design.