Stephi,
I can't help you with the Brimmer Plantation question, but I have been to Ocho Rios and definitely
recommend that if you decide to go ashore an organized excursion is the only way to go. Of all
Caribbean ports, I think those in Jamaica pose the greatest challenges for all but the bravest of travelers.
First, there is a relatively high crime rate. Second, there are literally a couple of hundred "hawkers" who will be right outside of the dock's security area trying to sell you things, take you on tours, etc. They are VERY aggressive and not for the faint of heart. The folks running the excursions shield the guests from most of that and your experience is a lot more pleasant, in my opinion. Third, the roads are very bad and poorly signed, so I would never think of renting a car and setting out on my own, which I have done on lots of other islands.
I have been to Dunns River Falls and enjoyed it. I would tell you, however, that it is fairly strenuous and isn't for the aged, handicapped, or those who are "balance challenged". . .
Enjoy your cruise.
Ted