Roatan is a fairly small island so nothing is terribly far from anything else. The Roatan Cruise Port is no exception, but at the same time there is not a lot to do within walking distance of the port.
Roatan also does not offer much public transportation. Your best bet is probably to hire one of the many drivers that line the street just outside the port. For about $70 for the day they will drive you around and show you some sights.
Roatan is not overly commercialized yet, so you'll want to bring your own snorkel gear and other equipment as there are not a lot of rental options.