I did similar to what you are planning - love Negril but want to check out what the rest of the island offers.
To answer your question directly - I think if you are in Ochi, Dunn's River Falls is a must do. Granted it's not as "natural" as Mayfield Falls - but it is spectacular. Kind of like Rick's in Negril - you have to experience it once.
Try to go on a day when there is not a cruise ship docked - as others have said.
Bring your own water shoes, and one of those disposable, waterproof, cameras - otherwise you will be paying for their shoe rental and a guide to take pictures from the bank with your camera.
Also, because Dunn's takes a good bit of time to do at a fun pace, I'd suggest you just take a regular taxi there.
Save the "hired driver for the day" for another day. Why pay him for the whole day, then have him sit in the car for several hours while you're doing the Falls?
A good trip to combine with the Falls, I thought, was Mystic Mountain. We did Dunn's in the morning, and MM in the afternoon. If you are staying at a hotel in downtown Ochi, MM is on your way back from Dunn's.
