It would be quite miraculous to bump into Queen Victoria or Mary on a cruise, and I imagine it would just about as unlikely to see Queen Elizabeth.
My wife suggests we may have seen some princesses on our last cruise but none were royals.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
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