There are many drivers with either taxis or mini-buses waiting outside the cruise terminal gates for the cruise passengers to take a tour to the local sites. Some will offer a set tour while others will tailor make a tour to cover your perferred options. The price of the tour will always come down in price if you haggle. Remember if you opt for a mni-bus tour the driver won't leave until it is full. I have been on both indepedent tours and those often on the quayside and without doubt the latter are much cheaper and just as good. The only time I won't use the local options is when there is a large deal of travelling involved, as you need to be back on the ship before departure and none of the islands you mentioned fit into this category. Just a word of caution don't visit too many sites in one island most of the islands have very similar excursions, (forts, rum distilleries, botanical gardens) so you can do similar things later on. Most islands are safe, if your driver is confident to take you to these attractions you are unlikely to encounter any problems.