We were on the Destiny in February for 2 five day back to backs. I used the gym most days and never had a problem getting on any machines. Only on one day did I have to wait for a treadmill, and that was only about 5 minutes.There were free weights available and a good selection of machines.
They have some free weights. Not sure if there is anything really heavy if you are into power lifting or bodybuilding.
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