ELTs have a close on 100% failure record which the industry seems to have willingly accepted
ELTs were never designed nor expected to work underwater. That's what ULBs are for.
In any case, there was a major study a few years ago that found ELT had a 75% activation success rate (not submerged).
And per SAR data the average search time for GPS equipped ELTs was something like 2 hours.