We have gone on Carnival's malibu excursion and had a good time. You get a tour of the factory and then some time to spend on their private beach. The beach has lounge chairs, umbrellas and a full bathroom and lockers area. Not much for snorkeling on the beach but the water is crystal clear and nice. I think you get one free drink and have to pay for the rest, it is definately not a free booze afternoon. The gift shop has pretty good prices if you are interested, the Lime Malibu is great and you can only get it in a few spots (Barbados being one of them).
You do not need to do a Carnival excursion to go to Malibu Beach. Just get offf the ship and get a taxi ( there are tons) and they will take you for $3 a person. When you are done there are plenty of taxis waiting desperately to take you back.