The ornamental design for a sail for a boat, as shown and described.
FIG. 1 is a front view of the sail for a boat, showing the new design;
FIG. 2 is a rear view of the sail for a boat;
FIG. 3 is a right side view of the sail for a boat;
FIG. 4 is a left side view of the sailboat;
FIG. 5 is a bottom view of the sail for a boat; and,
FIG. 6 is a left side view of the sail for a boat.
The broken lines showing the mast and boom outside the boundary of the sail illustrate the environment of the claimed design and form no part thereof. The broken lines within the boundary of the sail illustrate portions of the sail that form no part of the claimed design.