Or the magnetic field energizes the trace metals in the water? This is kind of interesting, and goes back to something I saw last year in National Geographic. They studied how giant sea turtles navigated over thousands of miles, and determined they use the Earth's magneic field. This could lead to advances in navigation way beyond GPS.
Pidgeons too. Their brains align to Earth's magnetic field and navigate accordingly.
Earth's Inconstant Magnetic Field
The pole kept going during the 20th century, north at an average speed of 10 km per year, lately accelerating "to 40 km per year," says Newitt. At this rate it will exit North America and reach Siberia in a few decades.
http://www.nasa.gov/vision/earth/lookingatearth/29dec_magneticfield.html