The closest airport to Harbour Island is Norther Eleuthera (ELH).
There are direct flights to ELH from Miami, Fort Lauderdale, Atlanta and Nassau.
The Bahamas Fast Ferry aka “The Bohengy” travels once daily from Nassau to Harbour Island as well.
From ELH, it’s a five minute, $5 per person cab ride to Three Island Dock and then a five minute,$5 per person water taxi to Government Dock on Harbour Island
Chatterbox and Jewelbox are within walking distance of the dock but cabs are available if you have lots of gear. Mainbrace is a short cab ride away.