The primary target of the ACTUV will not be Russian or Chinese nuclear-powered subs. Instead, it will be hunting the vast worldwide fleet of some 400 quiet diesel subs. Although nuclear subs can move with far greater speed and possess much greater range than diesels do, they aren't necessarily as quiet as diesels are.

This type of DRONE vehicle may also have its use in seeking out such underwater vessels used to transport illegal drugs and other contraband into the Country. Submersible's have become quite popular as a mode of stealth transporation, and appear to be used more and more these days for such tasks.
Ck out the ACTUV
http://defense.aol.com/2013/03/26/run-silent-run-deep-run-without-crew/?icid=maing-grid7%7Cmain5%7Cdl27%7Csec1_lnk3%26pLid%3D290022