Personally I never book ships shore ex, especially in places like the Caribbean where it is so easy to DIY. Do enough research and you will know where you want to go. Join with your fellow cruisers to make up a group and organise a private tour. Cheaper and far more personal. Google is a great place to start. Try 'cruise caribbean ports of call'.
Yes, I agree with Aldrich (why do www.CRUISE.co.uk persist in using surnames only for its contributors - very impersonal!?).
Unless you want something very special, like a helicopter trip, then do your own thing. There are shared taxi vehicles everywhere and the consolidator will form a group for you even if you do not have your own friends. Very much cheaper and more flexible.