My experience as an electronic engineer has made me very leary of electronic fail-safe devices. I'm a believer in the KISS principle and applying just the right amount of technology to solve a problem. Too much complexity harbors unforseen failure modes that are sometimes not discovered until the system is needed. And I don't want to loose a boat due to excess complexity. I would be much more comfortable with a simple wind-up deadman timer. Mechanical Clocks have been around longer than electronics and are well understood.
Qualified in Submarines 1965
"D..n the pressure, Six-Zero feet!"
"Most men would rather die than think, Most of them do!" - Bertrand Russell
"Boomers hide with Pride"