I would love to know how they do it at the Port of Miami. Would the boat be searched before leaving? Would you have to document the number of people on board..etc? Hmmm...still if someone had seen Dannielynn I'm sure it would have leaked out by now.
The last time I crossed the border, by land, I presented my voter registration card as ID. It was one of the listed acceptable ID's. I crossed into the US with no problem. I was kind of amazed that a no picture ID was acceptable.
I think a birth certificate is the only thing required for a minor child. At least, it used to be. Haven't border crossed with a child lately.