[dmurdoch]

This is only true in certain places. For example, in the thousand islands, the border between US and Canada is VERY nebulous. A line on a map exists, but you can drive your boat around the islands, entering and exiting the US dozens of times, and absolutely nothing will happen to you. The US Coast Guard is there boating around watching, but only rarely pokes around peoples boats.

Now I'm sure if you landed and somehow were talked to by the border police and didn't have a passport you'd be pretty screwed, but honestly thats pretty unlikely to happen. I know an old couple set in their ways who to this day just boat across the river sans passport for a favourite restaurant.