Howdy, Travis.
I didn’t do underwater stuff, but this was somewhat interesting. I doubt the claim in the article. Magnetic mines are detonated by the presence of a metal. A few feet of water will stop most particles of rays from a coronal mass ejection or fallout.
The particles of the solar wind and CME’s are charged, so most are captured by the magnetosphere. Thus they create auroras at the poles. These captured currents, like any electrical current, cause temporary intensification of the field. While much of the CME is blocked from the atmosphere by the magnetosphere, the resultant magnetic flux is not.