IDK about the particular mgmt operation on that ship, but any restaurant is going to have a certain number of empty tables that are awaiting reservations to show up. Obviously, they cannot seat someone at 6:30 when table is needed at 7:00. If you took reservations away entirely and made it all walk-up, then it is true a table would only sit empty while they cleared, changed linens and reset for diners. However, it may result in more people waiting. And, that would presumably erase the opportunity to keep same table and waiters each night - unless among the first wave to dine at opening.