The problem with notifying 911 of a potential problem is you have cast the liability on them, they are required now to respond immediately or as soon as resources are available and not terminate until a resolution is meet. Now you have taken a rescue squad out of service to sit until the paddler is back on shore. This rescue squad is now unable to respond to any other emergency that may arise. What you ultimately did was correct, wait for here to return. even though it was an inconvience to you, you felt a sense of responsibility and acted on it. Somewhere in heaven a star was placed next to your name.