Cheap ways to travel between Caribbean Islands?

My boyfriend and I planning to island hop between the caribbean Islands, starting at Puerto Rico, working our way South, getting back to the center and then heading North again. I have heard of mail boats and some ferries, but, more specifically what is the best (and cheapest) way to get from island to island?

The ferries are the cheapest, but you won’t get to all the islands that way. To see them all you’ll eventually have to take local planes. Frequently though, the price of airfare goes down as more people book the flights.

The following link might be more informative:

http://www.caribbeaninspired.com/caribbean/travel/island-ferry-travel.htm