The overall ornamental design for a diver's towable buoy, as shown and described.
FIG. 1 is a perspective view of a divers towable buoy having storage compartments showing my new design;
FIG. 2 is a top side elevation view thereof;
FIG. 3 is a top side elevation view thereof;
FIG. 4 is a bottom side elevation view thereof; and,
FIG. 5 is a left side elevation view thereof.
In the drawings, the broken line showing of a flag in FIG. 1, and internal structures in FIG. 3 depicts environmental subject matter and forms no part of the claim.