I agree with Bob C that the rental car is the best way to see the islands (we did so much more than the passengers who went on the tours and got to go at our own pace), but I strongly believe that reserving through the cruise line is the best bet because it saves you time, which is really precious while you are in port. We had others from our sailing who did it your way, and we saved between a half hour and an 1 1/2 hours each day by using the cruise line's shore excursion rental car. We know this because we compared notes with fellow passengers. When using the cruise line's services, the paperwork is done before you set foot in port, so you simply get your keys and that's it. No need to call for a shuttle, etc. The savings are only $5-10 per day to do it yourself, so it was well worth it to us to have the extra time in the islands.