It's not my understanding, if you're not anchoring or mooring or doing what they call rafting—I think you're familiar with that from the lake areas as well. As long as you're not putting down an anchor or tying up to some mooring spot in the river, you can conduct those kinds of activities.
I know there are areas where you have very calm water, for example, outside Rockport, Ontario, between islands, where many people like to do what you're suggesting. As long as they're not, for whatever reason, putting down an anchor or tying up to another conveyance, they won't be in violation.