the question needs more to it, if it is to be properly asked.

after all, the very measure of magnetic field strength (the oersted) is itself defined based on the strength of a magnetic field in a vacuum. (One oersted = the force of one dyne acting upon a unit magnetic pole in a vacuum.)

do you mean "are magnetic fields just as large in vacuums as in the ocean"? or "...as in the lower atmosphere"? or "...as in a mountain of iron ore"?

Bwa ha ha haaaa!!