It's well known that the earth generates its own magnetic field, but did you know that the sea also generates its own magnetic field?
A basic rule of electromagnetism is that any electric current generates its own magnetic field. So as long as the ocean flows, it will be creating its own magnetic field. This process is called motional induction.
Ion The Prize
"Ions" are charged atoms. When ions move through a magnetic field, they create an electric current. So what does that have to do with the sea?
The sea is full of ions, in the form of dissolved salts. And liquid water flows, so those ions are always in motion. Ions in the ocean flow through the earth's magnetic field, and that creates an electric current.