Well, are you experienced with cruising? If you did, you could pay less than that, about $80 per day, and go to several islands and do those things... you don't have to book an excursion with the ship. You go on your own, often using local transportation, which i have done because I have been to these islands numerous times... Cruising is a good option for that... especially in January.. rates are low... Western Carribean, or Eastern Carribean...
As an example, Some Western Carribean itineraries include the following:
Key West
Jamaica
Cozumel, Mexico
Belize --> Beautiful Snorkeling!!
Isla Roatan
Grand Cayman, Cayman Islands --> Beautiful Snorkeling too! 7 mile beach is only a $2 taxi ride
SOME Eastern Carribean includes:
Barbados (Sea turtles!! --> a $1/20 minute bus ride from the port)
St. Thomas --> crystal clear waters & amaxing beaches .. small enough to rent a scooter through town
Grand Turk
Puerto Rico
Dominica
Aruba --> just plain amazing!
There is just soooo much to do ... If you just wanted to go to a place in a resort, without breaking the bank, I would say St. Thomas... perfect mix of everything....