Thanks. I knew someone would know. Perhaps a redesign is in order? Hopefully they will find these. May the passengers and crew R.I.P.
Post 10 gives more detail.
Post 11 answers your question.
Thanks to both of those posters.
They normally make sounds underwater so they can be found.
I do wonder though if they can take the likely water pressures involved. Anything with voids or that is compressible will be utterly crushed at that depth.