As I've said from the beginning.. Carnival should accept more responsibility for the mechanical breakdown. No reason the customer should take ALL the risks. All along I've said a 15 - 25% discount on a future cruise would seem reasonable... but then Carnival doesn't listen to me!
BUT.. sometimes mechanical breakdowns are simply not forseeable.
I don't know if it's happened to you, but it has happened to me.........
Driving down the highway in a relatively new vehicle, and it simply breaks down. Do we keep a trunk full of extra parts to cover every possible situation?
Realistically no one can say they booked this trip SOLEY for the 20 -22 hrs they were going to spend on St. Thomas and St. Martin. They were also paying for the cruise experience that would fill all the other hours of a 7 night cruise.
So... seek fair compensation for the part of the cruise that you didn't receive.