The ornamental design for an outer sole of a boat shoe, as shown and described.
FIG. 1 is a top perspective view of an outer sole of a boat shoe showing our new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a front elevational view thereof; and,
FIG. 8 is a rear elevational view thereof.
FIGS. 1-8 illustrate an outer sole of a right boat shoe, with the outer sole of a left boat shoe being a mirror image thereof. In addition, the broken lines shown in the drawings represent portions of the outer sole of a boat shoe that form no part of the claimed design.