DCL recently changed their policy so you can do your online check-in at 12:01 a.m., same time the window opens for shore excursion booking. That's eastern daylight time.
If you don't have your passport handy, you can just fill in that you're using a birth certificate (unless you're going on a cruise that requires a passport.)
Online check-in will also ask you to list an emergency contact person; someone who's not cruising with you.
Any of your information can be updated/edited later.
When we did an online check-in last year it wouldn't accept our credit-card information because the card was due to expire a month before our cruise. I waited til we got the new card (the only change was the expiration date.) By the time I entered the new info and completed our online check-in, the only PAT's left were quite late. If I had known then what I know now, I would have just listed "paying cash" on the online check-in, then edited it when we received the new credit cards.