I can't speak for Carnival, but I will tell you what we saw on Princess in February 2002, just a few months after the awful events of September 11, 2001.
During the initial boarding process, your photo was taken digitally and stored in a computer system. Each time you left the ship, you needed your cruise card, which was scanned and you were obivously marked "off the ship". When you returned, you passed through a metal detector and/or security checkpoint. Your card was scanned again, and this time your picture appeared on a screen for one of the security team. This allowed them to visually confirm the identities of the people boarding the ship in a foreign port. The entire process went very smoothly.
I don't know this for a fact, but I'd be shocked if the cruise lines didn't employ "plain-clothes" security personnel in the public areas of the ship. Department stores do it to prevent shoplifting, the Las Vegas casinos do it to keep an eye on everything.
Frankly, the cruise line generated a lot less hassle than the airport security goons. Because of our unusual itinerary, our flights were all one-way tickets. Now, we all know that only terrorists book one-way tickets, which explains why I was virtually strip-searched at every airport. There's no easier way to sour my mood on my honeymoon than to make me take off my shoes and open my pants on every leg of my journey. We even saw the screeners confiscate a nail file from a passenger. The nail file was bent in half and tossed in a trash can. My opinion? If you can bend it in half, it's probably not going to be an effective weapon.
There are new security rules in place as of today, Jan 1 2003, which state that all luggage is supposed to be screened with the magic machines. Hopefully, this will speed up the process of getting through the airport - not to mention eliminating the embarassment of having a federal worker pawing through your underwear.
So, yeah, the cruise lines take security seriously, but they seem to do it a lot more gracefully and pleasantly than the airlines. That's not hard, but it is appreciated!