I think you need to have certain points pre-determined but remember if the area in question is always in a situation where things can go wrong. Then it just might do so. I know that the City of Niagara Falls Canada has a net which goes across the hydro canal which is swift water. This net assists the victims in need of help as well as the rescuers. They are always plucking victims out. The problem is by the time you are notified and set up as you state the person may have gone by already.