I'm sorry, but I haven't read the whole thread. I'm only responding to the intial post. I've been on 10 Carnival cruises and 3 others. I've always picked an excursion from the cruise lines. Some excursions are better than others, but I've never felt unsafe. Some have been better then I expected and some not as good as I had expected, but none were to the point that wish I hadn't picked it. If you have the attitude that you are going to have fun, then you will have fun. Again, I've never felt unsafe on an excursion.
There is also a plus side to using the excursions offered by the cruise lines even though they can be more expensive than what you can find outside the cruise line. The cruise lines keep track of all of their excursions, but they don't keep track of any that you might buy when you get there or online somewhere. So if for some reason something happens (like a flat tire or similar) and the excursion is late getting back, the cruise line will wait for theirs, but not others. That means if you go out on your own, there is a chance you can get left at the port.