I took the Cockatoo when I sailed on Carnival Conquest this past April.
It was WONDERFUL. A little more expensive - but a much smaller group. They also do a video on board and in the water which you can buy later ($40). They were even able to get it delivered to me on the ship before we sailed and we just got back in time for the last tender.
My friends all love seeing the video of the Stingrays. They really try to make sure everyone gets to hold one and get in the video. Also some good underwater video of the stingrays swimming around everyone and getting fed. They also add music to the video.
Just be sure to put on sun screen. No shade - you take off your shoes and sit right on the canvas of the catamaran. Fun looking down and wathcing the water rush by. Also a show watching them hoist the sails.
Look for Snowflake - really friendly stingray that just loves to be held. You'll know her by all the small white spots on her back.