then the submarine may have a use as a faster way of carrying out a 'quick and dirty' search for metallic debris on the sea bed.
Please can we drop the submarine stories? As many posters have pointed out, submarine sonars are not suited to listening in the 30-40 kHz range.
As far as magnetometers go, you cannot detect an engine-sized lump of steel in a depth of 4000 m, using a P-3's MAD or from a surface-towed magnetometer.
A submarine might be able to get one down to a depth of perhaps 500 m, but that still isn't going to help.