"Again excuse me if I am wrong, but magnetic fields can move through water can't they?"
Well, to my knowledge, magnetic fields pass through just about anything, including water, but how is that relevant? After all, these areas studied are formed of igneous rock. As the igneous rock cools, the iron grains in the molten rock alligns with the Earth's magnetic field. Once fully cool, these metal bits won't turn around. So, if at the time of eruption the field is varied just a little, it would show up in our studies.
"I remember an experiment in Chemistry class that we actually changed polarization and magnetism with water and heat(i believe)"
Number one, i don't believe water is magnetic, but i suppose that is possible.
Number two, These rock formations were formed long before humans heated up the planet, therefor the changes would be completely natural.
"And hasn't it been a known fact that the entire magnetic field throughout the world is slowly diminishing?"
Well, the diminishing magnetic field could be a sign of this switch. After all, you have to slow to a stop before you can turn change to reverse.