Beyond carrying weapons or higher-that-prosumer-quality video equipment, they are not all that different. UAS go from the sized aircraft one would typically think of as "military drone" to the small hand-launched aircraft used by people on the ground [0]. Using a submarine<->boat analog I'd argue with as the overall physics and operating principles of the two are hugely different.

[0] http://www.theguardian.com/world/2012/jun/12/us-military-min...