I was going to say the opposite
If your private excursion ends at the port I would think you will be fine. Back on board is usually 30 minutes before departure time. Check with your provider about the time and any guarantees they might provide.
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