What you are describing is active sonar. The downside of active sonar is that it also clearly reveals your own position.
As such, modern submarine warfare relies mostly on passive sonar - very sensitive microphone arrays (sometimes towed behind) to listen for audio signatures of surface and subsurface ships.
This means that submarine navies (and anti-submarine surface fleets to some degree) put a high priority on the ability to run quietly. A lot of work is put into things like propellor design to minimise the noise but there's a limit to how far you can go to eliminate cavitation and other noise sources. MHD, which would just be water flowing through a pipe, can potentially be much quieter.
As such, modern submarine warfare relies mostly on passive sonar - very sensitive microphone arrays (sometimes towed behind) to listen for audio signatures of surface and subsurface ships.
This means that submarine navies (and anti-submarine surface fleets to some degree) put a high priority on the ability to run quietly. A lot of work is put into things like propellor design to minimise the noise but there's a limit to how far you can go to eliminate cavitation and other noise sources. MHD, which would just be water flowing through a pipe, can potentially be much quieter.