You certainly do not need to purchase any kind of ships excursion to go to Paradise Island. It can be walked (depending on your idea of a walk),It is not a short stroll.
Water taxis, if you can call them that, run frequently from an area near the ships. They usually wait until they are somewhat full until they leave. For a few dollars per person you get a nice ride of about 10 minutes. Sometimes you will get a little "tour" offered by one of the crew, and occaisionally someone will come and sing a song or two for tips. This is definately the way to go.