This cache is hidden in the middle of Skull Island, the unofficial name of the largest island on the Bow River between Southern Stoney Trail and Deerfoot Trail. A sturdy watercraft will be needed to float the Bow River to Skull Island. Then beach on the shores of the island and explore - if you dare!
Skull Island:
If you're looking for a short float, start from the Sikome boat launch at Fish Creek Park (accessible from Bow Bottom Trail) and follow the river downstream to Policeman's Flats. It's easy to extend you adventure by starting further upstream (Glenmore, etc) or ending further downstream (McKinnon, etc). Expect some easy rapids, but nothing that can't be navigated by beginners in a typical watercraft suitable for the Bow River.
Have fun and stay safe!