by a comfortable margin; by a wide margin
Fixed expression used when someone wins or leads by a clear, decisive margin, often in sports or competitions. The phrase literally means 'opening the water widely', evoking a gap like a stretch of water between boats.
He finished first by a wide margin.
That team won the championship by a comfortable margin.
The expression likely originates from boat racing, where a lead is compared to the distance of open water between boats. The exact historical derivation is uncertain.