http://www.bahamas-travel.org/grand-bahamas-hotels/

How to get to Paradise Island/Nassau from Grand Bahamas?
Hello,
We are planning a 1 day trip to Bahamas from either Miami or Ft. Lauderdale. Can someone please tell me how long it takes to get to Atlantis Hotel in Paradise Island from Grand Bahamas? The cruise we are buying takes us to Grand Bahamas but doesn’t mention anything about Paradise Island/Nassau. Thank you in advance for your help.
I suggest you DON’T book that cruise if you are planning to stay on Paradise island.
Nassau/PI is 100 miles away from Grand Bahama Island, 2 very different islands.
There are no ferries or easy access between them, only regular scheduled flights (which would cost the same as if you just flew into Nassau from Florida in the first place).
There IS a mail boat that goes 2x a week and take 9 hours, they are oftentimes delayed or cancelled, so it’ s not a reliable mode of transport.
Either take the cruise and stay in Freeport (grand bahamas) or take a plane directly into Nassau and stay in Nassau/ Paradise Island.
BTW.. the bahamas is made up of 700 islands, so you have to be specific when booking a trip.