When it comes to brushed motors, there are many controllers you can buy that do forward and reverse for model boats.

These controllers work using a pulsed voltage signal to the motor. The longer the 'on' pulse the faster the motor spins and vice versa, the shorter the 'on' signal the slower the motor spins. The best way to think of this is by switching a light bulb on and off rapidly, by doing this the light appears dimmer because it's not been given enough time to reach the 'on' of the 'off' state.