As a guy who spends a week each year in Key West I would suggest you find alternatives that are the same as the Fury but not quite as expenisve since they are not connected to Carnival. You might not be able to do that depending on your schedule.
Also depending on the time of year snorkeling is not real great. For example if the weather has been just a bit rough you might end up disapointed. And certain times of the year Jelly fish are a problem.
With snorkeling catamarans there is lots of competition and most offer the same ie a popular reef and food and drinks afterwords.