Once you get to the pier you will pass through security with your boarding pass/tickets and government issued ID. Some itineraries will allow you to sail without a passport however we highly recommend that US Citizens always travel with their passports.
After security you will be checked into the ship, for this you will need your boarding pass, ID, credit card and you will be asked to complete a short health questionnaire.
The cruise line representative will then take your photo and provide you with your room key. This room key will not only allow you entry to your room but you will also use this for anything you would like to purchase while onboard the ship.