How is power consumption from a battery controlled?
The load will only draw the current it requires.

Taking something a bit more modest than a submarine, the starter in my car will draw a few hundred amps from the 12 volt battery when starting the engine, but the headlights will only draw 5 amps each, and the interior light will probably draw less than 1 amp from the same battery.

To control the speed of an electric motor, we often use Pulse Width Modulation (PWM) which turns the power off and on rapidly so the motor effectively sees a lower voltage when you want to run it slower.  

Alternatively, the motor could have separate windings for high and low speed - I have windshield wiper motors on my boat that work that way.