It is surprising that there isn't constant passive bathymetrics. The models exist, the hydrophones are already there, and the entire hardware/software stack needed to implement it is there. What's the hold up?
Just a guess, but stealth is one of the primary design requirements even passive coils are RF devices.
No way. RF doesn't go anywhere in water. Besides, an entire hydrophone array is at least 60 dB quieter than propulsion systems in terms of EM emissions and will be on par with life support systems, which I assume do not ever get voluntarily shut down underwater.
Can you elaborate on what passive bathymetrics entails?