Not to scare you, but in Jamaica, I would highly recommend a ship's excursion, or as Penny suggested, try shoretrips.com. The reason being is that Jamaica isn't the country you want to be roaming around in by yourself, which is why I recommend a ship's excursion, or an organized group excursion, such as shoretrips.
Be forewarned, in Jamaica, at Dunn's River if you go, merchants may hound you to buy. Part of the reason, I learned from my last cruise by our port and shopping guide, Ocho Rios only gets visited by 2 cruise ships per week.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas