Old type ELTs/PLBs transmitted a tone on 121.5 and generated so many false alarms that in the end they were largely ignored unless there was some other information to indicate there was a problem. These are now hardly used (maybe even illegal?).
Modern type transmit, on 406 MHz, a unique code tied to an aircraft or vessel so the authorities can ring you up first and see if you are in bed rather than swimming in the Atlantic. I believe these also transmit a tone on 121.5 so SAR can home onto you. Some also transmit a GPS position.
IMHO silly not to have a PLB when crossing water etc. The ones attached to the aircraft not so useful over water.