There is an easy way to get this, and then there is a fun way. Enjoy!
Please note - this cache is in a potentially dangerous location and seeking it is done at your own risk! I strongly recommend against doing this in the winter, as ice on rivers is always unpredictable. Legal entry points (PWAs) are provided, other entry points may be considered trespassing on private property. The terrain rating reflects the need to travel a significant distance on water.