It is possible that they are going to have the gangways open much earlier than 8 AM however.
We were on the Wonder on the South Caribbean itinerary a couple of weeks ago and even though the "official" arrival time at each port was listed as 8 AM they had the gangway open before 8 AM at every single port. There was a couple ports in which the gangway was open by 7 AM!!
So I would guess that they plan to have the gangway open before 8 AM and that your excursion will be given priority off the ship so make sure you are on time.