How close can you get to the top of Niagra Fall before you may go over the falls?

I sure do not plan to do this dumb act but am wondering at what point on the Niagra River would the boat start to be doomed and headed over the falls?

It would actually be very difficult to get a boat over the Falls at any point. The nearest launch upriver is above the control gates of the Hydro dams, which would catch most craft long before getting to the falls. Any heavy craft would bottom out before the falls as well, it is not very deep, and at night is extremely shallow as two thirds of the water is diverted for hydro. The most recent barrel attempts failed twice short of the falls as the craft ran aground, the third and last successful trip was aided by innertubes supporting the weight until the craft cleared the rapids above the falls. The water level can be so shallow it is possible to walk across only a few yards above the brink, a recent rescue there of a suicidal man was done by firefighters walking out to grab the man (tied off of course). Any boat in distress above the falls would be drawn ashore by currents, and Coast Guard advises staying in the boat, as it will have a better chance of grounding than just an individual floating. There has not been a reported boat going over the falls in recent memory because of this.

People have been rescued from boats and floats caught up almost at the brink of the falls by other boats secured by lines from the shore. It is possible (not a good idea, but possible) to take a reliable motor boat into the water not far above the falls where it is smoother and drive back out again. But the channel above the falls is pretty uniform and the current is constant and strong only speeding up somewhat as the flow goes over the hard material that forms the edge and is scoured clean by the force. The force also increases because of the island between the American and Horseshoe Falls cutting into the width at the brink.