once on board you can inquire about a waitlist, we decided one night to try the teppanyaki, stopped by around 5 asked if there was space for 2 sometime that night, she said that she would put us down and let us know. Turns out they had a late cancellation and we received a notification thru the hub app confirming our reservation.
Have you seen RCI Icon of the Seas... Massive ship, 250,000 gross tons, 1,200 feet long, 20 decks, 7,600 passengers and 2,350 crew that's 9,950 max capacity. I can only imagine 8000+ people trying to get on or off in port...