How did people in submarines get electricity?

Originally, a submarine's electrical supply was stored in large banks of batteries. The batteries were charged by generators turned by diesel engines when the submarine was on the surface. If the destroyers and aircraft could keep the sub down long enough, it would 'die' because of running out of power.

Today's submarines generate electricity using steam from the nuclear power plants. This allows them to sever their ties with the surface for extended periods, which allows them to stay submerged for many months.