The ornamental design for a pool island, as shown and described.
FIG. 1: Top right perspective view;
FIG. 2: Top plan view;
FIG. 3: Side elevation (the sides are identical);
FIG. 4: Front elevation;
FIG. 5: Rear elevation; and,
FIG. 6: Bottom plan view.
The broken lines in the drawing are for illustrative purposes only and form no part of the claimed design.