Staying at Harbour Island for a week in July and want to know the prices to get to Nassau for a day whether it by seaplane or boat. Which is cheapest?
Bahamas Ferries has ferry services that stop at Harbour Island, however it does not seem possible to do a round trip from Harbour Island to Nassau and back in the same day. They do have a day trip option the other way around (from Nassau to HI), but that's of no use to you.
I believe the flight is your cheapest option.