I have never been to Grand Turk so I can't help there.
Half Moon Cay is a great beach day.
Check out ther straw market in Nassau. It is within walking distance of the ship.
We love to snorkel but the water in February is a bit cool. We were actually there a year ago today and we did snorkel but the water was definitely cooler than we would have liked, about 72 degrees.
Enjoy! :beach: