But they refer to it as the slough, near Marymoor park.  And if KCSO "on the lake" is involved it's upstream (south) of the weir, I would think (as they launch from Vasa Park).  There's no islands there - check Google maps.  You should be able to walk out or have RFD reach it from the Marymoor bird path on the east side or through the brush to the Rowing Club or W. Lake Sammamish Parkway on the west side.