Guest Eliza, the P&0 liners, Costa, Holland America, and others, are all owned by the same company, Carnival, as posted previously.
I don't know if different quidelines exist for the different lines operated by this giant cruise liner company. But hearsay about conduct of the lines is suspect, if they are all under the same umbrella corporation.

New tankers (the better ones, anyhow) are double-hulled. Should cruise liners be built with this safety specification? What do you think, Charley?