While hours of operation vary from port to port, we've always found the same helpful and friendly (and patient) cruise line mbarkation people there to help in any way they can.  We always go an hour or two early, wanting to be one of the first on the ship. 

If you go early you may or may not find a place to sit; remember they'll be moving several thousand people through this area on Embarkation day.  Before that happens they'll have to get rid of those poor slobs who's cruise is all over.  You can tell them by the hang-dog look on their faces as opposed to your radiant look of expectation.  Their fun is over, yours is just beginning.  I go early to suck every minute of fun I can get out of a fabulous cruise that will go by all too  fast when the fun begins and you set sail.