You can carry on all your luggage on the ship if you want, they don't care ...but I'm assuming your talking about on a plane and that depends on the airline.
Recently on a spirit flight they made constant announcements of one carry on only, shopping bags no, making people try and get it in the test box to show it was the right size and even talking about the carry on weight limit. On another flight people were allowed on with a backpack and carry on bag with out a word.
If your worried just buy it after you arrive in your port town, less to carry anyway.
11 cruises in 2012, 90 nights at sea visiting 15 countries
8 cruises in 2013, 56 nights at sea visiting 7 countries
Next cruise: None scheduled
PM me if you need a cruisemate for any Europe, transatlantic, or Panama Canal cruises
Date, ship, cost pp