I just read up on it. Seems like they are chartering the ship the Norwegian Sky.
I've been on that ship and it's "nice" (not the newest one out there, but they have recently renovated it and aside from the fact that it doesn't have waterfalls in the ship's atrium and race car driving on Deck 8, you wouldn't notice any difference between the latest and greatest mega ships).
Norwegian Sky runs year round from Miami on 3 and 4 day cruises to Freeport and Nassau Bahamas. The staff is excellent, the food is great and Nassau is fun (you can take a ferry boat over to Paradise Island for the day).
Great Stirrup Cay is the other destination on the cruise and is a tiny little "key" in the Berry Islands chain of the Bahamas. They ferry you off the Norwegian Sky (if you want to go that is, you can stay on the ship if you like) and on the island they have a BBQ lunch, a very nice beach and you are free to roam the island where you can find some secluded areas and imagine yourself shipwrecked in the 1500s!
All in all, if you are going just ot party, then this "Groove Cruise" sounds OK.
If you are going primarily to visit Nassau and Great Stirrup Cay, then go a different time and you'll pay a lot less money.
One word of caution: In January, the ocean water will be COLD and the weather can be windy and chilly with highs only in the 60s and lows at night in the 40s.