i personally didnt love dunns river falls. Just my opinion. If you really want to see /swim with dolphins and wont be cruising again anytime soon..then go for it. I will warn you that if you do this and dont use your cruise ship to go thru, because it sounds like a long day, i'd be worried. I would only do both if you go thru your ship.
Not sure what port your landing at...but i know that from Montego bay to ocho rios is a very long boring bus ride.
I would wait and do the Dolphin Encounter at Chakkanaub (SP?) in Cozumel. It's a beautiful place. Dunn's River Falls is pretty but I think if we return to Ocho Rios, we will remain on the ship. The only port I've ever been afraid! However Chakkanaub is a beautiful place, an easy cab ride, and well worth it.
RCCL Majesty of the Seas
When we were in Ocho Rios, we booked (independently) a river tubing excursion on the White River and went to Prospect Plantation. The Plantation tour ran an hour over the published time, so I was a bit concerned with getting back to the ship in time, but these opportunities gave us excellent views of the rainforest areas and seeing "real" Jamaican life rather than the marketed Jamaica.
if you want to be waited on hand and foot, all the food and drinks you can handle, then go to the bamboo beach club. just a short walk from the pier and get there early and grab a beach bed just feet from the water.the cost is only $39.00 each for all day if you want , you can come and go as you please. all other excursions can be very high in price,this one is great for the money.have fun wish i were going back too!