As far as I know there are no longer any scheduled ferry services to Nassau from Miami or Port Evergaldes (Ft-Lauderdale). It's been tried in the past, but the operations were very short-lived.

My suggestion is that since you are coming this far, is to make it a point to visit PI via air. The hops are short and quite fast.. Of course there are 2-3 day cruises if time permits.

Enjoy, and welcome to Florida...

John