Dunnes River Falls is nice, but packed, and a complete rat-race if cruise ships have just unloaded into port! Wall to wall people. They were trying to herd us all up the falls in a huge line of dozens (if not hundreds) of people!!! We stayed at a fairly nice all-inclusive at Ocho-Rios (which we really enjoyed!), so Dunnes River Falls was right there. But, from MoBay, it would have to be a tour or a drive for you.
I much more enjoyed our tour to Negril! Nice beach. Our tour spent the afternoon there at at beachside grille type place. They were serving up big plates of jerk chicken or pork with Jamaican beans and rice. But before we saw how good that looked, we had just ordered a burger! We rented a jet-ski... the tour ended the day at that well known hangout (can't for the life of me remember the name of it now! A man's name....) where people gather to watch the sun set into the sea while they eat, drink, and be merry. This is the area where there are gorgeous rock bluffs that drop into the clear blue Caribbean, where people jump into the sea. This day was the highlight of our short trip. It has been years now since we went, and I still look at our absolutely INCREDIBLE pictures!
We stayed at a nice reputable place, and toured to Negril with a major reputable tour company. Both actually I think were American owned, at least they operate here in America. As long as you stay within these types of boundaries, you can have a wonderful and safe time!
I remember sitting at the pool with one of the several frozen fruity drinks that we enjoyed... A jamaican employ of the resort walked by and smiled and asked how we were doing... After only a day or so on the Island, I was comfortable enough to reply in the perfect Jamaican accent "Irie'Mon...Irie!!!"
PS: definately avoid smaller operations, and so called sight-seeing and shopping tours... They just take you to be hounded by locals to buy stuff. In this type of situation, tourist are just 'bait'.