It is enforced sporadically. My wife and I brought two twelve packs through in Miami in taped up 12-pack of pop case. One had 6 Mtn Dew and 6 Diet Mtn Dew, and one had 12 beers. They didn't look at the contents of either. We laughed and wondered if we could've brought more. Most cruises we bring either 6 or 12 twelve oz cans.
Once we had a 4-pack of 16oz cans and again, nobody said anything. We had meant to consume them in the hotel the night before, and forgot they were in the carry-on bag to be honest. Found them when we unpacked our bags in the room. Security never questioned or even opened the bag to look. I am not promoting this, just saying it happened.
Another time we tried to bring on a Russian Imperial Stout and a Barley Wine that were in 750mL wine-sized bottles (no other beer that time) and hoped we could pay a corkage fee to get one of them served to us for an anniversary dinner at Palo, and maybe one another night. Instead the bottles were confiscated at port... I tired to argue, but to no avail. Both the stout and Barleywine were well within the ABV (12% and 12.8%) of a wine, the bottle size was allowed. However the person read the rules as beer was 12oz only, and this wasn't specifically a wine in the wine bottles. So no go that time. It was a little disappointing, but perhaps I should've known better than to try.
And to another point, that time we were able to pick up our confiscated beer at the end of the cruise. It was at a table in the lobby just to the right of the walk-off gangway.
I say all of this to say, if you follow the exact rules you'll have no issues. If you try something else, your mileage may vary...