Apparently, the Navy heard an implosion on the first day the OceanGate went missing. So, the passengers likely died instantaneously.
"Sensors operated by the Navy detected the likely implosion of the Titan submersible hours before the U.S. Coast Guard publicly shared that it had gone missing — a revelation that means a five-day search that sparked round-the-clock media coverage may have been futile from the start."
https://www.npr.org/2023/06/23/1183976726/titan-titanic-sub-implosion-navy
"Sensors operated by the Navy detected the likely implosion of the Titan submersible hours before the U.S. Coast Guard publicly shared that it had gone missing — a revelation that means a five-day search that sparked round-the-clock media coverage may have been futile from the start."
https://www.npr.org/2023/06/23/1183976726/titan-titanic-sub-implosion-navy