From quick research I've found that a UUV capable of 4-5 kn, with the ability to run for 24 hours, and advanced detection and classification ability is a real possibility. Thus I'll probably tone the speed of mine down to 5 kn while extending the range to 120 nm (24 hour run time).
Adding some sources to the discussion:
http://auvac.org/platforms/view/148
http://www.militaryaerospace.com/art...k-pearl-r.html
http://www.navaldrones.com/knifefish.html