There have been several reported incidents lately in which cruisers on an excursion led by a tour guide have been robbed.

You can read about the two most recent ones on the following threads here on CLF:

Cruise Passengers Robbed in St Lucia (More Cruise Passengers Robbed, this time In St. Lucia)is being reported this morning, and the one in Nassau that made a lot of waves with some of the cruise lines at least temporarily halt the use of the new eco-tour when Cruisers were robbed at Nassau's Earth Village (Nassau robbery)

BOTH of these were on guide-led excursions. So, I guess we all need to realize that choosing to go with a guide - and even a fairly large group - does not necessarily ensure that you will be safe from crime.