I have not taken that particular excursion, but most of the snorkeling excursions are similar. You don't have to be a strong swimmer to snorkel. The life vest can be inflated or deflated to provide as much flotation as you need. The dive fins help a lot in moving around.
Most of the time you can just float face down and watch the fish go by.
We love to snorkel , give it a shot.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013