actually the ELT is designed for an impact on land. It's activated by a g-switch (no values given) and/or a mercury switch when position is changed into vertical (>70deg) . The ELT can be removed from his fixed position and is than able to float in upright position. Battery pack holds 48-60 hrs only. when no one removed an ELT from the fixed position, I think it sunk, and is destroyed by water pressure. Couldn't find an info for underwater ops, and it makes also no sense. The ELT has to contact a satellite to narrow down the position by 1.8 km accurance.