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.