Bluish skin, which is often described in watermen, is probably a sign of cyanosis - insufficient oxygenation of tissues with low-oxygen blood. One of the causes of hypoxemia is a limited amount of blood in the pulmonary circulation.
This occurs, for example, when there is a congenital obstruction in the outflow of blood from the right ventricle into the pulmonary artery when it is obstructed or narrowed. It is often part of a congenital heart defect known as Fallot's tetralogy.