The term 'Eagles of the Sea' is a nickname for the German Navy during World War I. The nickname was derived from the German naval ensign, which featured an eagle and a shield with a wavy line representing the sea. The crossword solution for this term is ERNS, which stands for the German word 'Ernst, ' meaning 'SERIOUSNESS' or 'EARNESTNESS.' This reflects the German Navy's commitment to their cause during the war.