The 8 rowers in a racing boat stroke so that all of the angles formed by their oars with the side of the boat stay equal. Explain why the oars on either side of the boat remain parallel

Assuming the side of the boat is a straight line, it would constitute a transversal crossing the lines of the oars. When corresponding angles at a transversal are congruent, the lines being crossed are parallel. Since the oars are those lines, the oars are parallel.