I have not been on this excursion, but remembered reading another post from another site about the horse back riding in Ocho Rios - thought you might like another point of view, here is what they wrote:
We did this tour during our thanksgiving cruise 2002. We were very saddened when we saw the condition of the horses. Very malnourished and obviously used continuously for tour groups without a lot of care given to them. The guides were very nice although they were not the owners of the horses. The horses were tethered in a large open area, appeared to be some sort of prior plantation area. We were then led across a VERY busy highway, with the guides running out in the street trying to stop the traffic. I hope y'all are very good at riding, because this scared me to death since we had to almost gallop across!! Then we were led through a kind of rundown area with gravel paths, a lot with huge cinder blocks and almost 45-90 degree inclines in places. One of the horses shoe's was coming off and the guides kept pushing it to go down these steep inclines, made me feel so sorry for these horses. We rode past homes that were nothing more than aluminum siding leaning on each other-kind of like 3 wall huts. If nothing else,my 3 teenagers(12,14,16) realized how some people have to live.
The ride down the ocean was unremarkable. If you don't mind getting your shoes and clothes wet, they take y'all way out in the water, the horses have to swim with the riders on their backs. We rode right through the middle of a resort area, with swimmers all over the place,they were angry. Horrible experience, the area is so poor and the guides obviously are trying to do the best with wha they have. Needless to say, they are very nice and keep mentioning that if you are having a good time they would appreciate any $$ you could give them at the end. We ended up giving them a big tip because we felt sorry for them. If I were you, I would bypass this excursion, we noticed that the horses and stables in cozumel were 100% better and well cared for. we plan to ride there on our next trip. Feel free to email me directly if y'all have any more questions.
I have found another website for Ocho Rios horse back riding: http://www.hoovesjamaica.com/beachride.htm
You can check out information about their horses, etc. Their ride is cheaper than the one on the ship, and seems to be better. According to the video on the website, their horses don't "swim" in the water, they run through the water. Kids under 16 have to wear life vests. They keep the saddles on as opposed to taking them off on the ship's excursion.
Good luck with your decision.