Stingray City in Grand Cayman is a one of a kind experience.
(confluence of a 3 foot deep sandbar with wild stingrays who have been hand fed by man, since the fishermen a 100 years ago while they cleaned their fish on the sand bar)
It is amazing to be in crystal clear, 3 foot deep water and to see 50+ rays swimming around you, constantly.
Did it in February and will be going back to do it again in January.
From what I have read, most of the other stingray excursions on other islands, have the rays penned up, instead of swimming freely like in Grand Cayman.
I would hold out to do stingrays in Grand Cayman and try another excursion, elsewhere.
I googled this out of curiosity, since I loved the one in Cayman. It looks like the Blue Lagoon and Blackbeard's Island have very similar tours. Whether or not the cruise line goes to those, I don't know. I would go to the Carnival website and look up the tour they offer. The full description button should give you enough info to tell. Good luck. This is one worth working for!