I'm not trying to defend the companies that advertise this. Rather, I want to make the user realize that the IP ratings are not considered when determining if something is covered under warranty -- that's the simple truth of how the fine print works. Warranties pretty much state that any kind of liquid damage is not covered, regardless of the IP certification. I agree that ads showing people using the phones underwater is misleading, but they usually get around it by including a little warning message along the bottom of the page or screen saying "Do not try this at home." Kind of like a car ad showing someone driving like a demon and drifting at high speeds, with a little message at the bottom saying, "Don't do this."
It'd be interesting to see how a lawsuit would pan out, but it'd almost certainly be a very quick slapdown of the plaintiff. If someone wants to try it, more power to 'em!