ELT(emergency locator transmitter) is activated upon impact. If the plane hit the water vertically going 500 mph or so the time of initial impact of the nose of the airplane with the water and the time the ELT(located in the tail) hit the water would be microseconds. Not enough time to get a signal out.
Now if the plane performed a controlled water ditching the ELT should have had time for at least a few seconds of transmission. Only problem is was there anything out in the middle of nowhere listening. It would also require more than one facility, satellite, ship, other airplane, airport, etc. to triangulate the signal in order to get a reliable location.
When it sinks, it starts a sonar pinging signal that can last ???? days before the batteries go dead. I think up to 30 days.
So if it is in the Indian Ocean and it is pinging..how long and how hard is it to find it?