If you are docking in Ocho Rios the falls are not that far away. I walked to them from the pier area in 1981. But that was 25 years and 100 pounds ago.
Seriously, it isn't that expensive and you will find no shortage of taxi and tour bus drivers offering to take you to Dunn's River Falls. Just be sure to settle on a price before you get in the vehicle and make sure that price is clear between you and the driver.