I think we're landing at Ocho Rios and not Mo Bay for starters. The trip from Ocho Rios to Dunn's River Falls isn't that far away. If you decide to take your own cab to the falls make sure you know the price before you get in the cab or they'll be demanding more money from you at the end of the trip. We were once there & an elderly couple took a cab to the Taj Mahal Shopping Center (we walked) which was 3/4 mile away & were charged $25 with the threat of calling the police if they didn't pay up.
At the falls I think you have to have a guide (again discuss they fee before starting). We've never done the falls on our own- always with a ships tour. If you go by cab to the falls you'll be entering & leaving from the top of the falls where a gauntlet of shops are located. After the climb (you depart mid way) you exit past the shops (there's no other way around) & that's where the hassling begins. The shop owners will fight over each customer like a pack of hungry dogs & won't let you go until you buy something from them. If you go on a ship tour, they used to go by boat to the bottom of the falls, then walk up the mid-point & return to the waiting boat by-passing the shops. There are no shops between. In any event be prepared to be hassled by anyone no matter where you go.
If we land at Mo Bay there's nothing close except a shopping area close to the ship & you'll have to do a tour or take a cab to see anything. The island does have it's beauty but unfortunitly it has many slum areas along with the high crime rate due to the poverty. Most of the locals are decent people who are only trying to make a living but there are a small miniority who give Jamaica a bad rap.
My suggestion is to deceide from the tour book where to go & stick whth a ships tour to avoid any hassle.