Hal offers most coke products. We buy soda cards on every cruise and I have no problem getting my diet coke.
Open seating allows you to dine several ways. You can dial 88 each morning and make reservations for that night, some will allow you to reserve for two nights. Or you can just show up between, I think, 6 and 8 and be seated alone or with others. If you opt for alone you may have a longer wait. Usually one is in and out in a very reasonable amount of time.