Here is DARPAs version of the exact same thing, from 8 years ago. https://web.archive.org/web/20081209121430/http://www.darpa.mil/sto/solicitations/underwatere

Here is DARPAs version of the exact same thing, from 8 years ago. https://web.archive.org/web/20081209121430/http://www.darpa.mil/sto/solicitations/underwaterexpress/UW_Express_Proposers_Day_brief_12-16-05_WEB.pdf https://web.archive.org/web/20081209121430/http://www.darpa.... https://web.archive.org/web/20081204131348/http://www.darpa.mil/sto/solicitations/underwaterexpress/index.htm https://web.archive.org/web/20081204131348/http://www.darpa.... http://adjunct.diodon349.com/Kursk-Memorial/Warpdrive_underwater.htm http://adjunct.diodon349.com/Kursk-Memorial/Warpdrive_underw... http://adjunct.diodon349.com/Kursk-Memorial/storm_over_the_squall.htm http://adjunct.diodon349.com/Kursk-Memorial/storm_over_the_s... There isn't any more information on it. Looks like that project was either shelved or a success and highly classified. I'd be willing to bed that it was cancelled because cavitation basically prevents any kind of stealth, which is the major advantage of underwater travel. Also there might be unknown disadvantages when slowing down, like the vehicle being ripped apart.