If you booked the excursion through Disney, you will report for customs at 6:30 a.m. (at least, that's what we had last week, when we docked at Crown Bay) and then you will have a specific time to meet onboard for your excursion, so you don't have to worry about what time they let people off. If your excursion is non-Disney, I don't know the earliest time we were able to get off last week, but the earliest meeting time for a Disney excursion was 7 a.m. Guests from decks 1, 2 and 5 reported for customs from 7 to 7:15 and those from decks 6, 7 and 8 from 7:15 to 7:30. I can't imagine that anyone would complain if you have a non-Disney excursion and showed up at 6:30.