Definitely don't need to do any 'Carnival' shore excursions in Ocho Rios, that's for sure! For example, it's much cheaper, quicker, and easier to do Dunns River Falls on your own. Just grab a taxi. As was stated, it's only 5 minutes down the road. That way you can stay as long as you want instead of doing it on their time. Plus, if you plan it right, you can get there when the cruise line's shore excursions aren't there and there will be a whole lot less people. Nothing more frustrating that being at the falls with hundreds of other people! We find out the time of the ship's excursions and go in between their times - makes for a much nicer experience.
And if you don't feel comfortable with a taxi (there are always taxis both at the pier and at the falls), then there are several wonderful independent companies, such as Shore Trips, that you can book - much less crowded.