Yes, that's about right. For an easier way to do it, see: http://www.npl.co.uk/npl/acoustics/techguides/soundseawater/content.html. It provides several alternative equations -- just press the "Interactive version" link next to your preferred one and punch in the parameters, it'll calculate the speed of sound for you.
I got 3,300 miles per hour using one equation.
So I'm very skeptical of the claims in the article. At the very least, it would take ENORMOUS forces to shove aside water fast enough to travel at 3000+ mph in the water. Not only is water quite heavy, by volume, but it's practically incomprehsible -- unlike air, you can't just "squeeze" it out of your way, you have to shove it aside, plus all the water that's all around it. That's why jumping off a bridge into a body of water is almost always fatal -- at high speeds, water is about as "hard" as concrete.
Cue the Nuclear Explosion image.