Remember Belize is a country. You will be in the harbor at Belize City. You will border a tender boat and take a ten to fifteen minute ride into port. In Belize check out independent companies like Coral Breezes or Ecological Tours of Belize.
We haven't been beyond the beach on Roatan so won't try to advise there.
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