Here is a short video on the advantages of cross training with rowing and cycling to complement both sports.
Though a little “hollywood” in some of its presentation (rain pouring over rower during his erg routine while looking over shoulder-:), it does point out a few facts about the two sports and muscle groups that are engaged throughout.
http://www.rowingnews.com/index.php?option=com_content&view=article&id=704