We have been to each of these ports and have gone off on our own. We have been to St. Thomas and St. Maarten about 6 times, the other 2 once each. We always go off on our own in St. Thomas and St. Maarten, and did so in the other 2 when we were there. I wouldn't feel unsafe. Just use common sense and do what you feel comfortable with.
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Have a great cruise. We have done the Southern routes 3 times, and they are fabulous!
Mariner of the Seas 3/30/08
"Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover."
Check out my cruise photos: http://community.webshots.com/user/Sheffie100