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.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013