What Dina said!
Here's the Carnival info about restricted items:
"Knives with blades longer than four (4) inches ( Recreational dive knives are allowed but must be held in the custody of the Guest Services Manager or Chief Security Officer and must be checked out/in by the owner for dive excursions during the cruise.)"
Here's the link: Frequently Asked Questions, Information | FAQs | Carnival Cruise Lines
Have a wonderful cruise,