The fast ferry service to Harbour Island is great. It can be booked on line and parking is easy. The ride is 3+ hours but its comfortable and the refreshments are ok. I suggest booking Voyager class for an extra $15 return, very comfortable cabin and free coffee.
My only gripe is that the timing only leaves just over 4 hours on he island, around 6 would have been nice. Book a golf cart from one of the suppliers, details are on the net
A great day out