Caribbean Cruise Lines and Bahama Celebration are real and legit cruise line and ship.
They do the two and three day Bahama runs. They have taken a couple of ships and brought them up to SOLAS standards and are now using them for "promotional" cruises.
While they are real I would stay away from them. Any "free" cruise that requires you to give them money up front is basically a scam and they will want more of you money later on.
Just skip it and go on a Carnival, Royal Caribbean or other short cruise. It won't cost much more and you know what you are getting.