I've used Shoretrips before, and they're pretty reliable. Similar to cruise line excursions, I assume they take a look at the local tour operators, and go with the ones that they think are the most reliable.
In Jamaica, I would highly recommend a reputable company, either through Shoretrips, the cruise line excursions, or another reputable operator. I wouldn't recommend going off on your own in Jamaica. The Dunn's River Falls excursion is the "Do Not Miss" type of excursion when you are in the Ocho Rios / Falmouth area.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas