Elt/elba
There are different variants of ELT's, broadcasting on at least three different frequencies (121.5, 243 and 406 Mhz), the 406 MHz now being fashionable since it is traceable by satellites and can modulate a registered ID and an optional GPS calculated position onto the carrier frequency.
However, there is a big difference regarding crash sustainability to blackboxes like CVR and FDR. The latter do not have to work after a crash, it's good enough if they keep recorded information.
An ELT however must work after a crash only. This means that it must be powered and wired to an antenna which in itself must not be obstructed by large metallic structures (faraday cage) or what's even worse - water.
Even the technically less burdened will understand that it is not so easy to construct and place an ELT that will be working after its holding structure crashed into the water (or into a rock) at a few hundred knots. An ELT is therefore totally useless for the purpose of tracking an aircraft that slammed into the sea. Black Box beacons that will work submerged are transmitting their pings in a way not detectable by satellites or flying aircraft.
However, it is possible and systems are commercially available to track an object (persons, cars, ships, containers, etc.) via satellite (and GPS position) as long as it it travelling. Then, in an emergency, the end of a track would indicate the position of the stricken object.