ADELT
In the rotocraft realm, there is a device called an ADELT, Automatic Deployable ELT. It is a frisbee sized device,about 6 inches tall mounted through the skin of the aircraft. In the event of a water landing, water sensors detect water in the aircraft and deploy the floating unit.
Several problems with large commecial aircraft, including deployment in flight, and the size of the unit creates alot of drag at high speeds.
In a rotorcraft (like those that transport oil rig crews) even if the tail breaks off and sinks with the ELT, and ADELT will remain near the cabin, that is equipped with emergency floats.