[toufka]

Many places consider the land below the high-water line of navigable waterways to be public (in midwest US at least). So if you're canoeing down the river and eat lunch on the bank, you're generally not trespassing. Smaller lakes are generally not considered navigable, though I'm not sure how it works for huge lakes like Michigan. I suspect a similar 'high-water' line is relevant.

You'd truly be pretty dense if you wanted to build a permenant structure below the high water mark.

[molsongolden]

This holds true in most of the west as well and provides access for river paddlers/floaters/fishermen.