Warp Drive Underwater in the May 2001 issue of Scientific American talks about the use of "supercavitation" technologies that make it possible for vehicles underwater (initially torpedoes) to go a few hundred miles per hour.
I thought the most interesting speculation was the effect of this technology on missile defense shields (e.g.. U.S. "Star Wars" initiative). Simply put a nuclear warhead on a missile that travels the first couple of hundred miles underwater very quickly, then pops up at the last minute to hit a coastal target.