You'll get a bigger and better response if you do some more typing and tell us where you are going...... Neil.
I hope you do not find Neil's somewhat curt reply unhelpful. It is possible to provide some general guidance.
There are trips which it is difficult or impossible to do on your own such as reef snorkelling, or long journeys to a particuler attraction. If you feel you must not miss these then you must book the ship's excursions. You will no doubt be aware that ship's trips are relatively expensive and if all you want is a shortish trip almost all Caribbean islands have a plentiful supply of taxis for you to arrange your own, especially advantageous if you befriend like minded cruisers.
Sorry if it seems curt but we are not texting here and if you look at most of my answers they are long and involved but if the question is vague then the answer is going to be too otherwise we are speculating and probably not giving the information required. There that took 2 minutes total. .... Neil.