I doubt seriously, that you'd be allowed to board the ship with the knife. I used to work, as a contractor, aboard Carnival ships when they were in port, in Tampa, and had to bring tools aboard, on occasion. I ALWAYS had to be very careful about what I brought aboard, even as an employee of the line! I think if you found a knife, or most anything that could be considered a weapon, the best thing to do would be to ship, or mail it to your home address from the port where you bought it. I think it would probably save you a lot of grief, and probably you'd never be able to bring the item aboard, and would wind up losing it!
Good luck, and have a GREAT cruise.