Well I'm not the most experienced cruiser and I've not been on a carribbean cruise, but I will offer some tips:

Excursions offer certain advantages. Since they are sponsored by the cruiseline, you will never miss the boat in the event that you are running late. They will hold the boat until your group returns. Since the excursion is in conjunction with the cruiseline, if you have any grievances and want to request a credit you are afforded a forum to complain by way of the purser's desk. It's a bit harder to do this by way of a tourguide . . . and they are less inclined to really care since at 5pm you're sailing away. Another advantage is the timing of the excursions. If you want to do a late or very early activity, it might only be offered by way of the cruiseline. Finally, shore excursions might offer a blend of activities that would be harder to find elsewhere (a bike and beach activity for example).

This said, about 100% of the activities offered via the cruiseline will be available on shore. 100% of the activities will also be cheaper on shore. You need to seek them out though (they arent in a handy booklet) and boat transfers arent guaranteed. Still, most port cities are easily navigable. It's nothing to grab a cab and just go.

I would NOT book an excursion for every port. I would though book an excursion for an activity which the group as a majority or a consensus is really dying to do. That way you know before hand that you'll be doing the "dynomite parasailing scuba snorkle acrobatic adventure" before you board and wont have to worry about what happens if you dont find where to do it.

Also, on cruises you get spoiled relaxing during at sea days and then shore days you get into tourist mode. I find not booking too much in advance will temper the self inflicted i want to see everything on shore instinct and keep the pace a bit more relaxing.

Flyertalk is one of the greatest spots on the web... but let's face it -- not for cruises. Go to cruisecritic.com or the other mega cruise site whose name I forget :/ for 1000 more people who are ready to give you every last detail.

BON VOYAGE!!!