Folks,
From Australian research of actual accidents, fixed ELT have a better than 95% failure to broadcast a useable signal, most commonly because the aerial or aerial cable is damaged in the crash sequence.
For aircraft that go into the water, the failure rate is 100%, the frequencies used for ELT do not propagate any useful distance in water.
Fixed ELT are a total waste of money.
This is why Australian does not mandate the carriage of fixed ELT, although it mandates an ELT on any aircraft traveling more than 50nm.