Gravity force is proportional to mass and the average density of submarine is close to water density so (possibly slightly less as positive bouyancy is preferab

Gravity force is proportional to mass and the average density of submarine is close to water density so (possibly slightly less as positive bouyancy is preferable for safety reasons) so gravity force produced by submarine should be very close to the equivalent volume of water. So instead of detecting big chunk of metal (which is still relatively small for the scale of gravity), sensor would have to be sensitive enough to detect mass distribution within submarine or the minor density difference due to slight positive bouyancy.