I would think that the water is quite "frigid"! How about getting him a small pool and setting it up in the backyard. Fill it with water, preferably warmer than the river and throwing toys in it. Otherwise, do what moondog has suggested. That is the way I taught mine....moondogs way. Don't throw the stick so far, just toss it enough for him to get his feet wet. Not all Rotties take to the water right away. My first Rott took some time, but once he got going, I couldn't get him out!
He'd swim 2-3 miles at a time, while I'd walk along the shore.