I've seen an explanation [1] that in water it depends on having two aerofoils — one in the air and one in the water (they should be called fluidfoils. I guess the one in the water, the keel, is acting as a hydrofoil.) At any rate, in theory, with two fluidfoils and fluids with any different velocities you can go at any velocity (i.e. in any direction, at any speed.)

[1] http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wi...

Yeah, they really do. I was looking into this a couple months ago when a friend at work, who has a sailboat, told me that they could do that, and that they could sail with a headwind!

Here's wikipedia's take on that: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

> Apparently sailboats do this all the time.

No, the article is about sailing dead downwind faster than wind. No regular sailboat can do that.