The open water, like river or something taking the flow into account is extremely dangerous. The flow may carry both the rescuers and victims away. Even the best swimmer, when swim against the flow could get tire very soon.

When I was younger, at my top, I could swim 100 laps on a standard pool. But when I swam at the downstream of a waterfall. With my best, I could only manage to stay at the same place. I was prepared and a rope on my waist, and after 5 min or so, I signaled my friends to pull me up. Without proper equipment, swim to aid should be avoided as it is really the last resort.