I've not done whale watching in Dominica. There government site is very good http://dominica.dm
They recommend Dominica Water Sports: Whale Watching in Dominica
Watch your available shore excursions, something may pop up 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