Victor Oquendo has details on the life-threatening heat wave impacting more than 200 millions Americans, and meteorologist Jeff Smith has the heat wave forecast; Perry Russom reports on the deadly boat crash off Miami Beach, where a barge collided with a sailboat from a youth sailing program, throwing overboard all five children and one adult on board, according to officials; Pierre Thomas has the latest on the urgent manhunt underway in Arkansas for a suspect who allegedly killed a couple while they were hiking with their two young daughters in a state park; and more on tonight's broadcast of World News Tonight with David Muir.