An argument between the occupants of two cars in Brighton followed a road rage incident, police said.
More than 40,000 runners were taking part in the 26.2-mile event amid temperatures of at least 24.1C (75.3F).
More than 40,000 runners have set off in the 26.2-mile event amid warnings temperatures may hit 23C (73.4F).
A search was launched after the man failed to surface while diving in Wastwater in the Lake District.
Lt Richard Sandford blew up his submarine under a viaduct stopping German U-boats in 1918.