[mcculley]

Could one use something like this from the surface to detect steel submerged under 20-40 feet of water?

[sllabres]

I think, not from the surface, but have a look here [1], where the author referenced from the IIEE article has build a submergible sensor and detected (a know) boat.

[1] https://alexmumm.de/pgProtonMagMarine_en.htm