UAV-towed gradiometer: magnetic signal decay with water depth
Seawater shouldn't have any effect, other than preventing close approach.

For equations you'd probably want Physics stackexchange.  Calculate the field pattern and gradient of an iron sphere or iron rod placed in a uniform 50,000nT ambient.

Are anchors that valuable, when compared to cosmic nickel-iron, especially pallasites?  Hug the ground, fly a raster across inaccessable public land (no roads, no junk cars etc.)  Any out-of-place iron objects would need a hike and a shovel, not rental marine salvage ship.



Note that "Gradiometer" is a magnetometer pair: the signal is DC from the b-field distortions caused by large iron objects.  (Clue: nanoTesla units.)  No, oceans don't block Earth's magnetic field.  Everyone search "OPM," latest cool magnetometer, like a SQUID but no cryo required, nor heavy shielding.  Laser plus rubidium vapor beats proton precession or Overhauser.  

But nobody has yet built a DIY hobbyist OPM instrument.  How far below a hexacopter must your magnetometer dangle, to avoid motor amps and stator magnets?!

Yes, this means Christopher Walken's helmet from Brainstorm (1983) doesn't need liquid helium vapor jets anymore.