I have seen the folks you are talking about. Some may work for actual tour companies or just be group organizers. I don't believe this is a position that the cruise lines have. Most cruise lines contract their shore tours with specific vendors and think those contracts are negotiated at corporate.
Our med cruise did have a "port lecturer". That is the dude whose job I would like to have.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015