I am so sorry you had such a rough time. I am sure the captain did not put the passengers, crew and ship into unnecessary danger. Carnival and all other cruise lines take passenger safety very seriously and at the same time try to meet as much of the scheduled itinerary as possible.
If you feel the need to vent go ahead and write the letter (I have also let Carnival and other vendors know when I am dissatisfied) but I don't believe you will get much of a different response than I have written here.
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, 2016, Royal 2017
Star Clippers: Royal Clipper 2015