EVERYONE has to report to immigration in St. Thomas.
If you have a morning excursion in St. Thomas you have to report between 6:30 and 7 am. If you don't, you have to report between 7 and 8 am. EVERYONE has to report by 8 am or NOBODY can get off the ship, and they WILL track you down if you don't report.
I am told this started after September 11.
I think immigration also finds this more efficient than having someone checking papers the entire time the ship. They can have someone there from 6:30 am to 8 am, and leave.
They did not do this on our Alaska cruise the year before. They continue to station an immigration official at the end of the gangway the entire ship is in port, and if you don't get off the ship, you don't have to report.