Remember the thirty minutes is the time you should already be on the ship, not the time to be arriving back at the pier.
when the itinerary says that you'll be in port until 6:00 pm, 5:30 is the time they start pulling in the gangway, and untying the lines. 6:00 is the time the ship is actually moving away from the pier.
Do not expect the ship to wait for you. Hubby and I can specifically remember at least three times seeing people left on the dock. Twice it happened on cruises when leaving Nassau (I think they forget to leave time to make all the way down that long L-shaped dock.)
Every time I've feft so sorry for them, running and waving their arms like that. I aways wonder if they remembered to take their passports with them, and if they have a credit card with them to buy an airline ticket, and a room for the night. They looked pretty miserable, standing there as we sailed away.
I do remember when we were on the Milli, one very pretty young woman, nearly missed the ship. The gangway was already partway up. They didn't put it down for her, but it was only about three feet from the pier, so one strong looking fellow hoisted her onto the ramp. That looked pretty scarey too. Somehow I think if she had been fat and frumpy they'd merely have wave her goodbye.